The Liberals are hoping that the young voters who came out in record-breaking numbers in 2015, helping the party win a majority government, won't stay home this year.

But their re-election hopes don't rest entirely on these youth voters — it wasn't just a "youthquake" moment four years ago that propelled Justin Trudeau into the Prime Minister's Office.

At 68.3 per cent of registered voters, turnout in the last federal election was the highest Canada has seen since 1993. Just over 17.7 million Canadians cast a ballot in 2015, an increase of about 2.9 million votes over the 2011 election, when turnout was just 61.1 per cent.

Young people contributed mightily to that big increase in voter participation. Elections Canada estimates that about 57 per cent of the population between the ages of 18 and 34 voted in 2015, up from just 42.5 per cent in 2011. That's about 1.2 million more voters. Post-election surveys suggest that the Liberals won the youth vote by a significant margin. Those voters played an important role in the Liberal victory.

But they weren't the only people who came out in droves. Turnout among people aged 55 or over was 74 per cent, beating their turnout in previous elections and boosting the total voter pool by 1.4 million compared to 2011. Their votes also played a part in electing the Liberals.

About 550,000 more people aged 18-24 voted in 2015 than in 2011 — but so did an extra 380,000 people over the age of 75. So turnout was up among both grandparents and grandchildren, contributing to the 4.2 million more votes the Liberals picked up.

There's a danger in overestimating the impact of the youth vote in the 2015 election, however. It was an important cohort and it may have been decisive in pushing the Liberals over the majority mark — but the Liberals can still win an election even if youth turnout dips back to traditional levels.

A tale of two turnouts

Canada is getting older. According to Elections Canada, people 55 or older represented 31 per cent of the population (and 38 per cent of voters) in 2004. In 2015, they made up 38.5 per cent of the population and 43 per cent of voters.

But despite the demographic trends, younger voters took up more space in 2015 than they had in previous elections. Though they had dropped to 27.5 per cent of the population (from just under 29 per cent in 2004), Canadians under the age of 35 represented 23.8 per cent of voters in 2015 — an increase of three percentage points over the 2011 election.

The election that gave Stephen Harper's Conservatives a majority government coincided with low turnout among younger voters. No other election since at least 2004 had seen young Canadians make up a smaller share of the people casting ballots.

So the 2011 and 2015 elections offer two very different turnout scenarios to consider as we look ahead to October — one in which young voters stay home and one in which they don't.

Two recent polls provide the perfect set of numbers for analyzing the impact of turnout.

Abacus Data and Léger published very similar surveys last week. The two polls were in the field together for much of the time, and both put the Liberals and Conservatives in a tie. Average them out, and you get the two parties deadlocked at 32.5 per cent nationwide.

But things change significantly when you break that down by age group.

Liberals hold wide lead among youth

Among respondents between the ages of 18 and 34, the average result for the two parties was 35 per cent for the Liberals and just 23.5 per cent for the Conservatives — an 11.5-point Liberal lead. The NDP trailed with an average of 19.5 per cent, followed by the Greens at 13 per cent.

The margin was tighter among middle-aged voters, with the Conservatives narrowly ahead at 32.5 per cent to 30.5 per cent for the Liberals. The NDP trailed with 14.5 per cent and the Greens with 13 per cent.

The Conservatives lead more comfortably among respondents over the age of 54, with 38.5 per cent support against 33 per cent for the Liberals. The NDP had 11 per cent and the Greens 9.5 per cent.

The numbers show why the youth vote is important to the Liberals: they hold a far wider lead over the Conservatives in this age group than the Conservatives enjoy among older voters.

But it isn't necessarily election-deciding.

Turnout's impact on the vote

By making some assumptions about what the population will look like in October (it'll be slightly older than it was in 2015) and applying the same turnout rates we saw in 2015 to these poll numbers, we see the overall result does not change significantly. We still end up with the Liberals and Conservatives tied.

That's because the higher youth turnout in 2015 brought that turnout closer to young voters' share of the population as a whole — which polls are weighted to match. So if youth turnout is much the same as it was in 2015, it probably wouldn't result in a boost at the ballot box for the Liberals over what the polls are showing now.

If youth turnout looks instead like the 2011 participation rates, however, the Conservatives get a boost. The national tie turns into a one-point Conservative lead.

That's not nothing. It could flip the result in maybe a dozen seats. In the current context, that could make the difference between the Liberals or the Conservatives winning the most seats.

But young voters are only one piece of the pie for the Liberals, and still the smallest one. If they're unable to get enough support from older Canadians — age groups they also won in 2015 — a "youthquake" of any magnitude won't be enough to save them.