As far as I observed, between 27-31, there is not so much skill pop or loss and after 31, players tend to lose skills. But it depends on the player, trainer, training type and maybe some other random factor. My 33 yo guard had a skill pop yesterday (outside defense) while my 34 yo forward almost lost all his skills in 1,5 season.
.. but I reckon around 27-28 your players start dropping relatively fast. (2-4 drops a season perhaps)