under age 未成年

A：Why not take me to the party, Mom?

B：You’re too young, dear.

A：Oh, I’m really as tall as you.

B：Oh, you’re still under age.

甲：为什么不带我去参加晚会，妈妈？

乙：你还未成年，亲爱的。

甲：不，我都快和您一般高了。

乙：但你还是没长大啊。