> Most kids in the US don't go to college

It's about 63% of high-school grads, including both 2-year and 4-year colleges: https://nces.ed.gov/programs/coe/indicator/cpa

From the same source: The overall college enrollment rate for 18- to 24-year-olds was 40 percent in 2020.

https://nces.ed.gov/programs/coe/indicator/cpb/college-enrol...

Many students will have graduated by the time they are 24, so it is possible that both statements 1. the majority of American children go to college and 2. 40 percent of 18-24 year olds are enrolled are simultaneously true

I’m not suggesting they these numbers are in contradiction.

I believe they are actually measuring any time enrollment in that period. Otherwise the statistic seems of little value.

On time high school graduation is achieved by around 86% of the population. This surely accounts for much of the difference.

https://nces.ed.gov/fastfacts/display.asp?id=805

Assuming (1) all college students enter at 18 exactly, and (2) all college students are enrolled for exactly the nominal time of their degree program, and (3) the number of students in each year of age is equal and enrollment rates are constant year to year, 63% of students attending college, and 40% enrollment of 18-24 year olds implies 37.8% attend 2 year programs and 25.2% attend 4 year programs. Now, the actual numbers are different than that because the assumptions are all three false, but the point is, that there is nothing inconsistent with the 63% immediate enrollment rate and 40% of students in the 18-24 year age band being currently enrolled.

> 63% of students attending college

63% is the statistic for high-school grads, whereas the person you're replying to is talking about all "18-24 kids", so "all kids graduate high school" is an appendix to assumption 1) you would need to add.