Generally juviniles are going to be more resiliant as they have not developed reinforced preferences yet. Older fish that have established territories and a set of learned behaviors in the wild will not adapt as readily to captivity and are generally more prone to developing repetitive or steriotypy behaviors or become aggressive towards tank mates. Please note I said "generally" individual personalities trump any reputation a species may have regarding behavior or any characteristics applied just to juveniles or mature specimens. Also note the vast majority, if not all, of the fish we get are going to be juveniles or subadults and as they become sexually mature changes in behavior should not come as a surprise.