For this season, for the purposes of the Premier League U-21 competition, U-21 is defined as being born on or after Jan 1 1994 (so all of Manu, Ince and Goldson are overage players, and Ankergren just misses the cut-off by 14 years).
This wasn't the FA Youth Cup - it was the Premier League U-21 Cup, a competition for Cat 1 academies joined by a number of Cat 2 and Cat 3 academies, at U-21 level.
The FA Youth Cup is for youth teams (ie players up to U-18 level). The round at which Albion enter hasn't been reached yet.