Some Under-21 teams have managers, but most don't.
Of course teams like England have good managers all round. They won't let you apply.
But you can easily get a job with say.. Ivory Coast U-21s or something. Just flip through the nations and take your pic and apply. You will most likely get the job.. I managed England U-21s in 2014. I wouldnt recommend going into young national managing, its very boring, and you always get spanked by Spain and Brazil, the wonderkid nations.