If you transfer free players, the max age you should buy is 31. from 32 they will retire once you sign them.
If you transfer from other player, the max age is 34, if he is on sale, ring that season, will not retire.
But from 32 to 34, expect them to annaunce retirement at the beggining of the next season.
The max age is 34, have never seen any older players.