From reading last year's registration rules:
Maximum 25 players, 2 of which are GKs, 8 of which are "locally trained" (slightly different from Premier League "homegrown" rules). Of the locally trained, up to 4 can be "association trained", meaning that they were at a different club within the same association. If you do not have 8, then that deficit gets subtracted from the 25 total roster spots
Locally trained = on the clubs books for 3 seasons between the ages of 15 and 21 ( Brandon Austin is the only player that qualifies)
Or
On another club's books within the same association (in this case, England or Wales) for 3 entire seasons between the ages of 15-21 (Maddison, Brennan Johnson, Solanke, Ben Davies, Djed Spence, Archie Gray, Ashley Phillips, Gibbs-white)
Any youth player born after 1/1/04 and has been with the club for at least 2 years since they turned 15 (or 3 years if they were loaned out for one year) can be registered with the squad without counting against the 25 roster count (i.e. Scarlett, Donley, Devine, Moore).