They have to be under 21 on 1 January of the year the season starts, so to be classed as under-21 for the 2023/24 season, the player must be under 21 on 1 January 2023, so born before 1 January 2002.
Jack Stretton and Ryan Rydel are both 21 but were born before 1 January 2022, so need to be registered this season.