The Cardinals lineup is old, but there is no need to worry.

As Derrick Gould recently pointed out, the Cardinals lineup is old.  The average age of the starting lineup is 30.5 (30.6 if Matt Carpenter replaced Daniel Descalso at second).  This puts the Cards lineup as the 4th oldest in the majors.

The projected starting lineup and age, is as follows:

1. John Jay, CF - 28
2. Carlos Beltran, RF - 35
3. Matt Holliday, LF - 33
4. Allen Craig, 1B - 28
5. David Freese, 3B - 29
6. Yadier Molina, C - 30
7. Rafel Furcal, SS - 35
8. Daniel Descalso, 2B - 26

Looking at the lineup, the Cardinals have 2 players, Beltran and Furcal, who are probably past their prime and for whom this is likely this last hurrah. With Oscar Taveras knocking on the door and Furcal's skills greatly diminishing, they will likely not be back with the Cards again next year.
 
If you also take into account that Kolten Wong could be ready to take over the second base job next season, then the Cardinals could get a whole lot younger by next season. 

Assuming that either Pete Kozman or Ryan Jackson (both currently 24) take over at short next year (for lack of a better option), here's what the Cardinals lineup could look like next year:

1. John Jay, CF - 29
2. Oscar Taveras, RF - 21
3. Matt Holliday, LF - 34
4. Allen Craig, 1B - 29
5. David Freese, 3B - 30
6. Yadier Molina, C - 31
7. Kolten Wong, 2B - 23
8. Pete Kozma, SS - 25

That would give the Cardinals an average age of 27.75, which would not fall into this year's top 12, according to Gould. 

The Cardinals been wise in doling out long-term contracts as well as drafting and developing players to replace their aging veterans (although the short stop position is still a question mark).

So, while other teams have concerns about their aging lineups (see the New York Yankees-average age 32.9), the Cardinals appear to have things under control.