It's normal in Chinese to say something like:
他大概三四岁左右 (source)
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".
他大概九十岁左右
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.