Did some friendlys with these 2, so technically I didn't spend that much greens the last few seasons on them. Normally I train a 18 or 19 year old player from 3-4 stars to 6 1/2 or 7 stars, then with 21 again up to that and then replace him with around 23-24 years old.
I would not recommend spending greens on players 22 and older, except he is maybe like your best player, then maybe up to 25. Then I would replace, its a waste of greens.