We have Davies, Lookman, Kenny, DCL, Vlasic, Beningame and Holgate who are all U21s. Throw in the players out on loan who may also be able to step up in a year or two (Dowell, Onyekuru Williams, Connolly, Walsh and maybe Galloway) and we've already got a huge amount of young talent to try and integrate into the first team.
Likewise half the players we bought during the Summer window would fit that profile also, Pickford, Keane, Sandro, and possibly Klaassen are all players who have yet to reach their peak.