Age as such is not a problem if we bother to find clear heirs/replacements for the older players before they are too old and retire. We can play Xavi and Puyol as long as they're fit, but to stay competitive we need to find and make out definite heirs early on. We have missed to do that for Puyol, which made us suffer a bit last season - now with the rise of Bartra that shouldn't be too much of a problem anymore, if he gets played as our no 1 defender and not as a subsitute for Mascherano. Still, relying on Puyol as our "new signing" instead of buying fresh defenders or giving the youngsters more chances much earlier was a bit of a gamble. As for Xavi, we have candidates such as Roberto and Samper, but neither of them are currently getting enough playing time for them to grow into the position and for us to be certain that they will do the job. Xavi is still younger than Puyol and should have a few years, but we must find a good replacement now if we want to avoid having to rely on him even when his fitness and "quality" levels go down and, thus, a little slump. He's still hugely important for us and performs well in most games, but waiting till he doesn't anymore will be dangerous as it might be too late then.
That, however, is not solely down to Tata but more to our management in the past few years.
(And no, I don't mean there are one-to-one replacements for Xavi and Puyol or that Roberto is the next Xavi etc etc)