Know How I know you're old?

Basically, I'm looking for clever responses to this question. My boyfriend and I had just watched "40 year old Virgin" and we started teasing each other with "Know how I know you're old?" jokes. I'm just curious as to what you all can come up with so i can use them in the future (I'm not as quick-witted as he is). Plus it's just fun stuff. Thanks people - you guys rule!!!