He is actually born after the u21 cut-off so he should qualify as u21 but you only need to look at the squad list UEFA revealed to see Denayer is club trained. It has 16 foreign players, 5 HG and Denayer, no u21's.
So he must be club trained.
This article
states:
http://www.manchestereveningnews.co...news/denayer-lead-youthful-charge-man-8930063
Kompany has long been a fan of, and a support to, his countryman. And when Denayer’s loan spell at Celtic ends in June, his development will be assessed, along with fellow loanees Karim Rekik, aged 20, and 19-year-old Marcos “Rony” Lopes.
All three qualify as home-grown and club-trained players, and may get the chance to go on the club’s pre-season jaunt to Australia – those decisions have not yet been taken.
However, the most articles are saying no club trained players.