I've seen managers (farms) that train their players even when they are 31-32yo, sometimes even 33.
For me, the problem is not that players reach their potential in their 30ies, the problem is, especially for SF's, that they'll only play 1-2 seasons on their natural position, before starting to decline. The rest of their career, they are usually playing either on C or PG, which is totally ridiculous comparing to real life.