depends. If you let things go, you'll lose ranking spots, tho ranking in U21 doesn't mean swat! ranking in U21 gives you an idea how strong ( or weak) last seasons generation was... nothing more nothing less. a rankings 70 U21 could end up ging undefeated, if the happened to have a very good generation for a season. Likewise, a top 3 U21 team could end up losing 6 of their first matches, because their generaiton is very poor for that year of the U21... (it's unlikely tho, since the top nations often have large ammount of players, which gives you a larger pool to select from...)
as for younger players: if they are as strong as the 21y olds, then sure, you ought to take younger guy's... However, if that isn't the case, then why bother taking younger players? They don't make your squad stronger, and if they aren't as strong as the 21y olds, then it's unlikely that they'll (the 20y olds that is) be substantially stronger when they are 21y olds themselves.