"Something to keep in mind: as in the real world, younger players will improve more quickly than older ones, and taller players will train faster in some areas while shorter players improve faster in others. You'll have to take advantage of this information to optimize your training routine."
I think this tell us that you CAN in theory train a player until 39 but that it will be substantially slower than an 18 year old even if both players are "generated" in the same season.
Hope this helps
-NS