Depends tho.
i.e.: I turned 21 this year and honestly i changed a lot mentally since i was 19, not so long ago. And i'm pretty sure that probably when i'm around 23 my mind will have changed a lot again too.
So no, age is more than a number (in some cases).
But there's some people who can't simply grow up, so there's that too. For them is just a number.
![deadbanana](http://img.photobucket.com/albums/v232/korn0818/ATRL_Smilies_All/bananas/deadbanana2_zpsd45da7d4.gif)