I am basing it on the influence a person's environment has on them. At the workplace or university, a person is more likely to encounter influential peers and mentors.

Telling me a developer is likely to get the same kind of experience working bit jobs as a contractor in high school as they are either doing the same while pursing a CS degree, or working in a development team is preposterous.

Edit: I just realised you're the OP. Add "gaining some maturity" to the things you'll get from the above.