Number fun

There's a logic to this apparently but most of all it's fun.
I never realised this until someone pointed it out recently.
Let's say you were born in 1970. Your mother was born in 1945 and your father in 1940.
When you are 40, your dad will be 70
and when you are 45 your mother will be 70.
This works for all years of birth within a century, ,01 through ,99.
Once you cross the ,00 border the age of the older person is the year of birth of the younger person (last two digits) plus 100.
Example: you were born in 1975 and your child was born in 2002.
When your child is 75 you will be 102.
Funny, isn't it?