Youth failing in training

There are youth players in 'B' Team or youngsters in main squad who look talented, have good attributes for their age and some of them perform consistently at a professional League, but it seems that by an early age (19-21yo) their attributes seem to decline.

Does that show a good but lazy player or should we assume that this youngster will never devellop further and look to get rid of him?

It depends, at that age they will need quite regular playtime (a few starts here and there and lots of sub appearances), so give him playtime and put him in your first team for better training. If his personality is poor then tutoring and game-time.

Also make sure to upgrade coaches and facilities.

If he's not good enough for your team, then tutoring with regular sub appearances (use reserve fixtures to keep him match fit) or loan him out.