As with all decisions, it depends.
For example, I bought 3 key older players to get a leg up on you back in our DV.3 days. Once my trainees got better than the two big men, I sold them off for more or less what I spent on them. I kept a PG around until this season when I no longer had Cup games in which he could play effectively. His skills had begun to drop, but Solid passing and handling are still valuable in the lower leagues so he went there for about half what I bought him for.
And don't overlook the real value of the Experience rating on players. Older players with RESPECTABLE and higher EXP do outperform their ratings.
I think most BB managers don't appreciate or understand the value of experience in this game.