Question

Can the ratio of a parent's age to his or her child's age ever be exactly 10 : 9 ?

Answer 1

Yes, if the parents had the child at a very young age. ex. Parents: 80 years old Child: 72 years old

Answer 2

Mathematically, yes.
Realistically, no.
The difference in their ages is the parent's age when the child was born.
Let's say that the youngest possible is 12.
Then the ratio would be 10:9 when the parent is 120 and the child is 108.
Not likely.