When buying an older player you really have to know what the future plan for the player is. You should know in advance that you're either going to use him as a temporary ringer and sell after a season of play or you're going to hold onto him until he's ready to retire. There really is no in between point because a 28 year old player's value will have dropped significantly by the time he's 31 and it won't really be worth selling at that point (unless he's one of the best BB players ever). If you sell after one season chances are you'll still be losing money on the player but you won't suffer too badly.
That's my opinion on buying older players at any rate.
when i watch the tl i would say, a 25 year old player would decline significant in the price till he is 28 and when you buy in the same "quality range" i believe the absolute loss is less with the older guy(which is also cheaper so you could get some extra succes with the rest of the money).