Not necessarily kids who got held back (some are), but because of US History of states' rights, every school district in every state has a different cut date for their school year grouping, which makes things overly complicated. So a lot of these mid-later 2010s are naturally the oldest kids in their grade, rising sophomores in HS. I love how Canada's federal system means the school year grouping is the same for the whole country and they go by the birthyear, meaning all 2010s graduate HS in 2028, all 2011s graduate in 2029. Much simpler system, wish we did that.