[pron]

First, college is used as a proxy. The significant drop coincides with the drop in participation in industry, the data is just cleaner.

Second (and unrelated to the discussion, really), I don't know where you get your assumption that learning to code in college is too late. I've been in this business for twenty years and some of the very best developers I know only learned to program in college. If anything, I'd say that not going to school at all and having a weak background in algorithms/mathematics is a much greater stumbling block for some software achievements than not programming before school, but even that is probably a bad generalization. Excellent developers come from all backgrounds.