There is so much that raises questions. I guess though from what I have gathered through readings is some of the newer more dominant strains are having a greater impact on the younger crowd than original strains. Also have to consider that for a large while not having in-person schooling and places where those in that age group tend to hang out (bars, clubs, etc) probably helped to limit the spread from that group.
I always thought this for vacinations...take care of the 65+ crowd and essential workers first then go to the 18-30 crowd because (I thought) it would be pretty obvious that group would be responsible for spreading.