Can 九十 = "ninety" also mean "nine or ten" (like how 三四 means "three or four")? 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.
No, since 九十 already means exactly 'ninety', it cannot be use as an approximate figure for 'nine or ten' 

'nine or ten years old' has to be written as "九至十岁"

You can say 十一二 (11 or t2) , 十三四 (13 or 14) or even 十五六七 (15 to 17) but not 九十 (9 or 10)