Lyric discussion by silverplate 

Cover art for Twenty-One lyrics by Cranberries, The

It sounds to me like an angry song too. The image of her kind of showing her teeth and saying: hah, I'm 21 now, I'm mature and strong, you've got nothing on me now; kind of contrasting the melody, which makes it so interesting.

But then again, it could also be about facing the horrible feeling of getting old, and trying to stay 21 forever.