That depends on how you play the game. If you *must* be promoted every year, or go for winning every competition then no, age doesn't matter. But if you play for fun, and don't mind staying in the same league for couple of season then leveling up young players becomes not only good, but great. You preserve your team's strenght (actually level up it) while not having the need to spend any money. That means that by them time your team will need new players, or you'll aim for the top, you'll have a whole lot of tokens/money to play with.