(CNN) Nancy Pelosi and Chuck Schumer didn't do any favors for Joe Biden with their televised response to President Donald Trump's immigration speech last week.

Pelosi in particular has drawn praise since the mid-term election for her skillful maneuvering to secure the House speakership and her aggressive jousting with Trump during the government shutdown. But the stiff and widely panned televised response from the two leaders -- an instant target for late-night comics -- quickly reopened questions about whether the Democrats' national leadership is too old. That's not a helpful backdrop for Biden, 76, as he nears a decision on whether to seek the party's presidential nomination for 2020.

A familiar face vs. someone new for a youthful coalition

Debate about the age of the party's leadership is likely to grow more heated over the next year, both as that nomination process heats up and as Democrats in Congress intensify their confrontations with Trump. On both fronts the party faces the same paradox: almost all of the Democrats' most familiar and powerful faces are graying while it is growing more reliant on younger voters to win elections.

That disjuncture is likely to sharpen questions about whether Democrats can maximize mobilization of their increasingly youthful coalition without a leadership that better reflects it. Heather Hargreaves, executive director of NextGen America, the group founded by liberal billionaire Tom Steyer to engage more young people in politics, says she believes that in reaching younger voters, the party's message is more important than the age of its messengers.

But she adds, "Do I think that in the long run the Democratic Party needs to make more changes? Yes. There has to be some accountability for the fact that young people are the ones that are catapulting these people into office. And they should reflect that."

The Democratic advantage among younger voters has become an indispensable component of its electoral strategy, especially as Trump has steered his message so narrowly toward older, blue-collar and evangelical whites.

In 2016, Hillary Clinton beat Trump by 14 percentage points among voters younger than 45 while losing by nine among those who were older, according to the CNN's exit poll.

In the 2018 mid-term election, Democratic House candidates significantly expanded on her advantage with the young: they carried fully two-thirds of voters 18-29 and nearly three-fifths of those aged 30-44, according to the exit polls conducted for CNN. That was the best performance for Democrats in both age groups for any House election since at least 1986. By contrast, Republicans held a narrow advantage among voters over 45.

A millennial wave

The sweeping Democratic gains last November significantly lowered the age profile of the party's House membership. Before the election, according to figures provided by Drew DeSilver, a senior writer at the Pew Research Center, Democrats had only one Member (Hawaii's Tulsi Gabbard) from the Millennial Generation (which Pew defines as young people born from 1981 to 1996) and 43 more from Generation X (born from 1965 to 1980). That means the two youngest generations accounted for less than one-fourth of the House Democratic caucus, an even lower percentage than the roughly one-third they comprised among House Republicans last year.

But the November results swept in a torrent of younger Democrats . In the new Congress, DeSilver has calculated, the Democratic caucus will include 16 Millennials and 72 members of Generation X. The two youngest generations now account for over 37% of the total Democratic caucus (about the same as their share among Republicans). Baby boomers dropped from about three-fifths of House Democrats before the election to just over half after; the older "Silent Generation" fell from about one-in-six to one-in-nine.

The generational split is not an ideological split

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The generation gap among Democrats is often equated with the party's ideological split, but the latter doesn't consistently track the former. The Democratic Millennials in Congress include Rep. Alexandria Ocasio-Cortez, who has rapidly become one of the party's most outspoken liberals, but also Conor Lamb of Pennsylvania and Collin Allred of Texas, two rising young centrists. In the 2020 presidential field, the oldest possible candidates include two of the most prominent centrists (former vice president Biden and former New York City mayor Michael Bloomberg) as well as two of the most ardent liberals (Senators Bernie Sanders and Elizabeth Warren.) "Both age and race do not correspondent cleanly to ideology," notes Matt Bennett, vice president for public affairs at the centrist Democratic group Third Way.

Instead, the differences between older and younger generations in the party may revolve more around accessibility, the style of communication (with younger generations much more fluent in the new forms of social media), and differing assumptions, rooted in differing generational experiences, about how aggressively Democrats can pursue their goals without provoking a backlash, especially on social issues. One reason some critics, for instance, are dubious about a Biden presidential candidacy is it would reopen wrenching debates from the Bill Clinton era over the party course on issues such as crime and welfare at a time when it was much more dependent on blue-collar white voters.

A grey tint in Democrats' leadership

US Speaker of the House Nancy Pelosi, Senate Democratic Leader Chuck Schumer, House Democratic Whip Steny Hoyer and Senate Democratic Whip Dick Durbin appeared at the White House after shutdown talks with Donald Trump.

In picking their leadership for the first Democratic majority in a decade, the party did not ignore these generational changes. But they made almost no accommodations in the inner circle of the most powerful positions.

For their top House leadership, the party chose Pelosi, 78, as speaker; Steny Hoyer, 79, as Majority Leader and James Clyburn, 78, as Majority Whip. These are the same leaders Democrats had in the exact same positions when they held the majority from 2007-2011.

Younger members were elevated into leadership this year, but only for positions one step below: Ben Ray Lujan 46 as assistant speaker; Hakeem Jeffries, 48, as Caucus chair; and Cheri Bustos, 57, as chair of the Democratic Congressional Campaign Committee.

The imbalance was even greater in the selection of committee chairs. Democrats chose the most senior party member on each standing House committee as the chair for the new Congress; in no case did they bypass seniority in selecting a chair.

The result is an unmistakable grey tint on almost all of the most powerful committees. The new chairs of the Judiciary (Jerrold Nadler), Natural Resources (Raul Grijalva), Homeland Security (Bennie Thompson), Budget (John Yarmuth), Education and Labor (Bobby Scott) and Foreign Affairs committees (Eliot Engel) are all either 71 or 72. The chairs of the Energy and Commerce (Frank Pallone), Oversight (Elijah Cummings) and Ways and Means (Richard Neal) committees range in age from 67 to 70. At Appropriations (Nita Lowey) and Financial Services (Maxine Waters), Democrats have picked chairs that are 81 and 80 respectively.

The biggest exceptions to this pattern are Adam Smith at Armed Services, who is 53; Mark Takano at Veterans' Affairs, who is 58; James McGovern at Rules, who is 59; and most prominently, Adam Schiff at Intelligence, who is only 48. Pelosi also appointed relatively younger members to lead two select committees: Kathy Castor, 52, on climate and Derek Kilmer, 45, on modernizing Congress.

But even with these exceptions, House Democrats have created a situation in which party leaders born before the Korean War will be the face of almost all of their major confrontations with the Trump Administration. And that could lead to more grumbling among the party's expanding ranks of youthful office-holders.

Pelosi agrees to some changes, others may be on the way

Pelosi, as part of the deals she negotiated within the caucus to regain the speakership, agreed to establish a three term limit on the party's top three leadership positions (which can be extended to a fourth term by a vote of two-thirds of the conference.) Those limits would be applied retroactively, which means Pelosi, Hoyer and Clyburn, who all served in their current positions when Democrats last held the House from 2007 through 2011, could only remain in them for one more Congress after this one.

But some leadership sources expect that when the Democratic caucus votes on these proposed changes later this winter, younger Members may push to impose similar term limits on committee chairs, which would create more turnover. House Republicans have operated with such rules since 1995 and have also been much more willing than Democrats in recent years to bypass the most senior committee members when choosing chairs.

The irony is that it was Democrats who initially changed their rules to select committee chairs through a party vote, rather than rote reliance on seniority. That change was imposed by the huge Democratic Watergate class of 1974, and they used it back then to immediately oust three elderly committee chairs. But Democrats in recent decades have grown much more reluctant than Republicans to bypass seniority in selecting their committee leadership.

Norman Ornstein, a political scientist and resident scholar at the American Enterprise Institute, says that despite the generational divide between the Democrats' leadership and membership, he does not believe the seniority system faces as great a threat among Democrats now as it did then.

The principal motivation for change in the 1970s, he notes, was that many committees were chaired by conservative Southerners who consistently opposed the Democrats' national program, but who accumulated seniority because Dixie in those years still would not elect Republicans. Today, the party has many fewer ideological outliers, and the seniority system has created a wide range of committee chairs who are African-American (Thompson, Cummings, Waters, Scott), Hispanic (Grijalva) and gay (Takano). "It's like the old saw: I didn't believe in the seniority system until I was here for 20 years," Ornstein notes.

But while the seniority system hasn't prevented the Democratic leadership from reflecting the party's diversity in most respects, the glaring exception to that rule is age. And Ornstein says that the party's older chairs will likely face close scrutiny, particularly since cable television is likely to cover many more of their committee confrontations with Trump officials than in earlier administrations. "There's going to be some pressure on a lot of these chairs to perform now," Ornstein said. "They are going to get a lot more visibility."

Similar issues are certain to emerge as the Democratic presidential nomination process gains steam. Many of the party's best-known possible contenders are older: Biden and Bloomberg (both 76); Sanders (77), Warren, who has already declared, (69), and Sen. Sherrod Brown (66). Another group of declared and possible candidates are much younger including former Housing and Urban Development Secretary Julian Castro (44), who announced his candidacy last weekend; former Rep. Beto O'Rourke (46); and Senators Cory Booker (49), Kirsten Gillibrand (52) and Kamala Harris (54).

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Age isn't the only factor that determines whether a presidential candidate can energize younger generations. During the 2016 Democratic primaries, Sanders dominated among the young, though that was against an opponent also in her late sixties who had long struggled to project the authenticity that many younger voters prize.

What is clear is that maximizing turnout among younger generations is central to Democratic hopes in 2020. In two years, Millennials will exceed baby boomers as a share of eligible voters for the first time in a presidential election, and the first post-Millennials, an even more racially diverse generation born after 1996 or after 2000 depending on the definition, will be eligible to vote. Those younger adults remain deeply alienated from Trump. In this week's latest CNN poll , conducted by SRSS research, among adults aged 18-34 just 29% approved of Trump's job performance and twice as many opposed his proposed border wall (63%) as supported it (just 31%).

That discontent over Trump's direction virtually ensures strong turnout among younger voters in two years, Hargreaves maintains. "I think young people will still turn out in high numbers even if the Democratic nomine isn't the perfect nominee in their mind because of Trump," she says.

That could prove correct. But one of the biggest decisions facing Democrats is whether they can count on Trump alone to energize the younger generations central to their fortunes, or whether they would be better off with a nominee, and a party Congressional leadership, that more closely reflects the coalition they now rely upon.