OK, My Theory.. Take your age..
Add 15 years for being married, or, 10 years for a long term relationship.
Add 10 years for each child you have.
Subtract 1 year when they turn 18, and 1 more when they graduate high school.
Add 5 years if you have a mortgage payment or rent payment.
So, I am 43
Married +15
3 children +30
2 over 18 -2
2 graduated HS.. -2
This makes me "feel" 84..
Nothing scientific.. just something I have joked with my wife about for the last 22 years.