It's normal in Chinese to say something like:
he is probably around 3 or 4 years old
This is called Approximating with sequential numbers on the Chinese Grammar Wiki.
However, 九十 is problematic because 九十 ordinarily means "ninety".
he is probably around 9 or 10 years old
he is probably around 90 years old
Question: Can 九十 also mean "nine or ten"?
It may simply be that we're not allowed to use 九十 to mean "nine or ten" (i.e., it's an exception), or perhaps "nine or ten" vs. "ninety" is usually inferrable from context.