To sell or not to sell?

[Toxcatl]

90% is not that much, not at all. It is possible to train the team up to 130%+ without paying real money (being in Gold1 or better in Platinum helps).
21 years old is almost an old man. Not yet, but close.
The offer is very good. I'd sell and buy 18 y/o FT instead. Transfer market resets every 4 minutes or something, so it's not that hard to find a good player if you put an effort to it, -- said the guy who is trying to buy good 18 y/o ST for three days, lol.