David Henderson nobly expresses interest in my proposed bet:

I might take that bet. But I don’t believe on betting on a change in a
number without having a good idea of what the number is now. Please
tell me the number and link to your source. That way we can agree on
the source. Then I’ll tell you if I’ll take the bet.

More than fair, David. 

I propose that we use the official numbers from the National Center for Education Statistics’ Table 212.  2009 is the latest available year of data.  29.6% of 18-24 year-olds were enrolled in 4-year institutions.  I bet that in 2019, that percent will be no more than 10% lower.  Rounding in your favor, I win if the number is 26.7% or more.  If the number is lower, you win.  If the data series is discontinued, the bet is canceled.  Stakes: $100 at even odds.

I’m happy to tinker with the terms.  What do you say, David?

P.S. Thanks to Gordon Dahl for pointing me to the NCES website.

Update: David accepts in the comments.  Another bet I hope to lose!