In childhood, there’s a tendency to want to be older. Many a 4-year-old would rather be 6. For those who are only 9, it sounds bigger and better to say they’re double-digits 10.
Ditto for teenage-hood — the older you are, the more sophisticated and grown-up it sounds to say you’re older than you are, old enough to get into a restricted movie, perhaps. “Under 17,” after all, still requires an accompanying parent or adult guardian