You would have to create an U21's league in the editor, as well as U21 sides for all the clubs you wanted to play in it, which would probably mean recreating the entire national ruleset and league setup for England, which is one **** of a task.
Chelsea Reserves play in the Reserve league anyway, why not just limit your reserve squad to U21's only and sell/release/loan any players who are too old, or promote them to the first team squad if you need them as backup?