it all depends what position the player is playing and how much important is he for your team..
most of the managers would say to sell a player when he hits 32 but I would say that if a player hits 32 but is an important player of your team the why let him go.
as far as the skill drop is concerned,for me,it varies from player to player.I have managed a 36 y.o player for half a season(he was my regular starter) and did not get a single skill drop but at the same time I have seen a double drop by a 32 year old player of mine.
If you ask me the age I would say-
Starter C-32
starter forwards-34
starter guards-33
for backups you can keep a player till he hits 33 and then replace him.