Yes, started volunteering at 16, got an offer soon and went fulltime. Dropped out of university. At some time I was working on two jobs, one part time of course, and did some freelance while still working full time.
I don't want to speak for Kessel Run, but citizen: probably. Secret clearance: if needed, they'll pull you through that process and that will be a thing of value you get from such a job that you can put on your resume for future jobs (clearances are typically good for 10 years).