Not all the factors are the same. It used to be legal in all states to leave school at age 16, which is part of why the traditional workforce participation stat

Not all the factors are the same. It used to be legal in all states to leave school at age 16, which is part of why the traditional workforce participation statistic is for ages 16-64. But that only remains true in 16 states; 34 states have raised the minimum school-leaving age, yet the workforce-participation statistics are still calculated for age 16-64. Naturally you would expect lower workforce participation among people aged 16-18 when it's illegal for them to drop out of school and get a job instead. It's also now more de-facto expected than it used to be that you attend at least some university (2 years of community college if not a 4-year university), so workforce participation rates for people aged 18-20 are declining for non-mysterious reasons. Although that's at least something you can legally choose not to do.