Most commonly referred to someone's age. It is said to be one's "Prime," age, where they look/shine/feel their best. Sixteen is the age where you get multitudes of exciting privileges such as quickie-driviers' license, get a decent job (Host/hostess,) and you're no longer at the very "bottom" of highschool.
Daughter: "Oh, Daddy, please can you get me a convertible when I turn sixteen?"
Daddy: "Of course, Sugar plum!"
Daughter: "Oh, thanks Daddy!"
Wife: "Didn't I just look so good when I was sixteen?"
Husband: "Yeah...This photo book sure has some crazy pictures of us crazy teens!"