Bryan Caplan wins if, in 2025, the fraction of 18-to-24-years-olds enrolled in four-year colleges is greater than 27.0%. The latest reported number, for 2021, is 29.1%.

From Bryan's bet wiki: “We are betting that when the 2025 data comes out, Samuel wins if the fraction of 18-to-24-years-olds enrolled in four-year colleges has fallen more than 10%.  I win if it's fallen less.  Since the current [2015] rate is 29.9%, he wins if the rate is 27.0% or less.”

Notes:

(1) the blog post announcing the bet gives a different, wrong target percentage. I've checked with this with Bryan, who says the bet wiki is the authoratative one

(2) still, it seems to me that 10% down from 29.9% should be 26.9% not the wiki number 27.0%. I imagine that Caplan is just being generous

(3) The data source used for the bet is here (see column 4).