England’s national under-21 football team, also known as England under-21s or England U21(s), is considered to be the feeder team for the England national football team. This team is for English players aged under 21 at the start of the calendar year in which a two-year European Under-21 Football Championship campaign begins, so some players can remain with the squad until the age of 23.
https://en.wikipedia.org/wiki/England_national_under-21_football_team

