FT and ST train just fine for older guys. Therefore it would make sense to train them when the rest of the options have become very slow or when the player is capped.
Stamina has diminishing returns. Even if you think high stamina still gives an improvement in efficiency for a player, you need to consider how much more difficult it becomes to manage GS on a high stamina player. When you realise it's hard to manage minutes on a high stamina player and also determine what is the optimal minute range, you will not care at all about that small improvement in performance, I promise.