Depends its all ways fun to get a baby fish and grow it up but with some fish like Vieja cichlids babies are totally drab and colorless and its difficult to pick out a nice one so its better to wait till there juvies ( or buy 10 of them )
I tend to get them as young as I can. I got my 2 Oscars at less than half an inch and have grown them out to 12"+. It's been great watching them grow and I find they are much more responsive to me. Even I don't live at my parents anymore, where the Oscars are kept, they still go mad at the glass when I go over to visit!
Babies and juveniles. It is fun to watch them grow. Full grown are usually more expensive also. I have seen adult oscars(12"+) that were as cheap as younger ones. I assume the store was having a hard time selling them.
Babies or Juvies...depending on the fish. I'd buy a baby Pbass because they grow quickly and are pretty hardy for little guys. And they're cheaper But most of the other fish, I'll buy as Juvies since they're still small and passed the survival hump of being small babies.