A childhood belief about family

When I was 11, I used to believe that the way to tell how old your friend's mother was, was to add up the ages of all her children and add that figure to 25 (all mothers had their first child at 25). I was so confused because some of these mothers (by my warped logic) were 36 (if they had one child) and some were almost 100 years old (if they had loads of kids). I could not work out how my best friend's mother still looked 25, but by my own special system was actually 92.