you would have to start a new game as normal then go the main squad of the nation (eg england) then make a new user (you) with the original user sack the u 19 or 21s manager continue until on the news it says he is sacked then still on the first user go onto your profile(the one with no team to manage) offer the job to yourself then accept whatever role you offered yourself then you will have the job of the under 19 or 21 (depending on what team you want to go
hope that clears it up for you