Is it because people or so young, or that the pipeline to become a junior-but-still-responsible person goes through schools rather than something like an apprenticeship.
Both, I think. The field has grown in size, so that alone makes the young-to-middle-aged ratio skewed. Then, many middle-aged programmers drop out to become managers.
I totally agree that programming would be better taught by an apprenticeship model than the college model, and I say this as a person with two college degrees.