I too prefer to buy younger fish for several reasons. Younger fish seem to be more stable as far as changing water conditions, you get to watch them grow, it allows them to pair off as they would in the wild and gives you a choice of the ones you really want to keep for breeding (if your going that route).
On the other hand if you buy older fish, then your possibility of not having to wait until they mature is much greater. But on the down side to that, half their life is already over.
IMO young fish are the way to go, you get a good run for your money and they are generally cheaper.
