For much of the past decade, the Philadelphia Phillies had one of the best offenses in baseball. But that was not the case last season, and the biggest question facing the Phillies this season is whether the offense rebounds, or continues a regression that could threaten to leave them on the outside of the postseason for the first time in six seasons.

Philadelphia’s offense began to make some noise in 2002 and 2003, and in 2004, the core of their playoff teams began to take shape. Jimmy Rollins, Placido Polanco and Pat Burrell were already on hand to start the season, and then Chase Utley came along in May and Ryan Howard in September. The group would finish in the top six in wOBA in 2004, 2005 and 2006 before punching up to second-best in the game in 2007. By 2007, Shane Victorino was on hand, and ’07 was the season that Jayson Werth stormed onto the scene as well, with a nifty .385 wOBA. That season, their wRC+ of 107 as a team was fourth-highest in the game. It would also be their pinnacle as an offense, as would be their .354 wOBA.

While the offense slumped the following season — their World Series championship season — they came raring back in 2009, placing fifth in both wOBA and wRC+. But the past two years have seen the start of a decline. To wit:

Year wOBA Rnk wRC+ Rnk 2007 0.354 2 107 4 2008 0.337 8 99 t-11 2009 0.340 t-5 104 5 2010 0.328 t-11 100 t-11 2011 0.316 15 96 t-13

After placing fifth in both categories in 2009, the team dropped out of the top 10 in ’10, and the descent back to the middle of the pack continued unabated last season. And the Phillies have done very little to bring new faces to the mix. Yes, they were smart to wait out Jimmy Rollins and get him back in the fold given the other alternatives, but the team’s other prominent free-agent signees on the position player side — Jim Thome, Laynce Nix and Ty Wigginton — aren’t going to bring much to the table. Thome should be good, but back in the National League he will have a limited role, and neither of the other two compiled a wOBA better than .322 last season. Any improvement will have to come internally.

Looking at xBABIP, there is some hope that the Phils will see better results this season:

Player BABIP xBABIP Diff Rollins 0.275 0.320 0.045 Utley 0.269 0.306 0.037 Howard 0.303 0.322 0.029 Polanco 0.292 0.322 0.029 Nix 0.284 0.312 0.028 Mayberry 0.293 0.315 0.022 Victorino 0.292 0.311 0.019 Ruiz 0.308 0.306 -0.003 Pence 0.361 0.322 -0.039

Aside from Hunter Pence, that’s a whole lot of BABIP underperformance. There’s a good chance that some of those numbers will even out this season. But will they be enough to turn the Phillies back into a powerhouse offense? Doubtful.

The other problem with the lack of fresh faces is that the returning faces aren’t all that fresh. As Joe Posnanski reminded us last week, the chances of putting up a truly elite season go south once players go north of age 30. And that’s a big problem for the Phillies. The team has led the Majors in batter age in each of the past two seasons, and they figure to be at or near the top of that list again this season. Using Baseball-Reference’s method for calculating batter age (which is just at-bats + games played) and cribbing together both Bill James and FANS projections for the 14 Phillies hitters on the 40-man roster that figure to get the lion share of playing time, you get a team batter age of 31.92 years. That’s older than the club’s league-leading totals from 2010 (31.8) and ’11 (31.5).

The 31.92 figure includes 335 at-bats from 24-year old Domonic Brown, which at this juncture is probably a bit optimistic. The only other under-30 batters in the group are Pence, Michael Martinez and John Mayberry. And while Mitchel Lichtman points out over at the Book blog that the curve may need to shift to the right by a year to a year and a half, most of the Phillies hitters would still be on the down side of that slope. Victorino is 31 this year, Howard 32 and Rollins, Ruiz and Utley 33. The new guys aren’t much help either. At 31, Nix will be the youngest.

So will an aging offense that is already in decline drag down Philadelphia as they hunt for a sixth-straight playoff berth? Looking at the rest of the division, the Mets pitching will remain a cover-your-eyes mess, but the Braves, Marlins and Nationals will all feature formidable rotations, especially if Tommy Hanson, Josh Johnson and Stephen Strasburg turn out full seasons (or in Strasburg’s case, as full a season as the Nats will allow him). The Phillies might not end up facing elite pitchers every time out, but they didn’t last year, and still turned out a below-average offense.

Only one of the 32 teams to reach the playoffs over the past four seasons did so with a team wRC+ lower than 95. If the Phillies offense continues to decline that is a bar they may very well drop below. If they do, it will make it a lot harder for Philadelphia to reach the postseason. Their big three starters should be exemplary once again, but Joe Blanton may not be as good as Roy Oswalt and Vance Worley will be hard-pressed to duplicate his rookie success. Combine that with an offense that ain’t what it used to be and improvements from the Nationals and Marlins as well as a still-strong Braves squad, and the National League East race might be a lot more hotly contested than it was last season.