Can 九十 = "ninety" also mean "nine or ten" (like how 三四 means "three or four")?
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)