Who is old formula

I was talking with a friend who just turned 37 yesterday. We were talking about how as you grow, being ‘old’ seem to be a moving target.

We managed to find this simple formula to find out who is old:

For a child of 4, 20 seems old
When you are 16, 40 seems old
When you are 25, 50 seems old 
For my friend 60 seems old

The nice thing about this formula is that when you are 100 you know that you are old