I didn't say a 26 years old is valuable to train, I said I'd train him if it was for my NT :)
I know that this season an old player from our NT, 25 years old (well last season) or 26, had 3 popups :)
but I generally wouldn't train past 24
then again, depends on your goals and league and everything
It depends whether you want to create a monster or a MONSTER!!!!!!!
With the players 23-26 likely to be your key starters I would say that giving a couple of them training on secondaries can be useful. I wouldn't add primaries to a 25yo if I could buy a 20yo and he needed the improvement more but an already valuable first team player with just 3 pops a season in Passing (example) could be very beneficial to a team vs. just adding more JS to new yth coming through. Depends also if you are able to train for team improvement or whether you need to train for economic return.