Let's separate the question from politics (legally operate).
My friends and I regularly "stole" our dads 32' sportfisher under the guise of sleepovers at each other's houses for overnight fishing trips to an island 30ish miles offshore. We started doing this around 12 or 13.
We cleaned up the boat and dad never knew until he came down, found the boat gone & reported it missing. We were 17.
Then, there are the many young teens - of both sexes - who have circumnavigated in craft smaller than I would take on.
I really didn't boat alone until recovering from many years of racing sailboats. I felt I had "lost" overall skill after so many years sailing the same positions on teams of 10 or more.
Eventually, I cruised my powerboat alone for five years on the Pacific Coast.
It really depends on the person, their parents, and desire to boat alone.