I personally prefer to buy grown fish straight out (more affordable in the long run), although have raised about half my collection after getting them as babies (more affordable in the short run)...
I like buying them small for the most part. It's cheaper and it's always fun to see how much they've grown. But I guess it also depends on the situation. If I have big fish and want to buy a new fish, I would have to buy a grown fish.
If I can I buy them young -but as others have said- it depends what tank they are going into. Been there and done that with too small fish into the wrong tank...
atm Ive got a discus setup in one tank and Ive had them since they where about an inch. Now they are about 3. Ive also just started up a 55g with 10 F1 angels that are about a penny in size. I really like that.
In some of my other tanks it would be murder to put young ones in
i dont see the point in buying an adult.
its just more fun buying young fish.
you get to see them grow up.
have more experience w them.
plus its easier to manage cause maybe you have a small tank that couldnt hold an adult.
but you get to enjoy it while its young.
For along time i wanted a big aggresive male red devil. I bought several mature fish that did not live up what i was looking for. in the end i bought 5 babies and let them pair up. after breeding a couple of times i got the fish i wanted and he was awsome.