my view (as a non-spender) is all or nothing... target one or two 18yo FT then build them up and keep using greens on them (assuming they perform of course). IMO better to continue using greens on the 7 star 25yo who will still stay in your team for a number of seasons than waste them on the three young 5 stars.
sure, the young players will gain much more from each green but what's the point if they'll be gone by the end of their first contract, probably earlier, and replaced by some newer version of themselves?
i have a 27yo 7 star who is still my best, or at worst equal best player, who has played over 400 games now. i have only just stopped/dramatically reduced in the last season or two feeding him greens. that's largely in part to finally finding another 95%+ FT 18yo at the start of a season a couple of seasons ago. he's now 7.5 stars at 20 and i feed him instead. the rest basically get nothing unless required due to scheduling.
i would also add don't be afraid to stockpile your greens until you find the right player. no point wasting them on someone who will be trasferred in a season or two. find the right player and look after them and you'll have them as the backbone of your team for 10+ seasons (all without spending a cent).