There is no "perfect" time to advance an Age because there are so many different ways to play this game.
Just one example: if you want to go the fighting route, then you need to find an Age in which you understand what the various units are capable of and what combination of units works best for various circumstances. You'll want to level up all of the "basic" Attack GBs (Zeus, Aachen, CdM) and maybe a few of the not-so-basic ones (AO, TA, Kraken, HC, etc.). And then there's the Arc to consider. Depending on your FP production, that can take quite a while, so it had better be an Age which doesn't bore the stew out of you.
And even then, given all that time and effort, you still might not be the "big dog" in your Neighborhood because chances are excellent that at least a couple of folks followed the same path you did, only they started weeks, months or even years before you did and so, until they Age up, they're going to be well advanced of you, even though you're both in the same Age.
The "best" time to advance is when you feel you've accomplished all that you wish to do in a certain Age.