This isn't all that uncommon. The same was true of both my wife and of me; we both have Fall birthdays, and started kindergarten before we turned 5. I turned 18 several months into my freshman year of college.

FWIW, I don't recommend this, but (and I've faced this with my own kids now) there's really not much choice. I was already bored in school; if I'd been back another year, it would've been even worse.