When you ask about age, you will use 几岁 or 多大.
When you use 几岁 or 多大?
If a person is under 10, then you use 几岁. Otherwise, you use 多大. I am not 100% sure, but apparently, it is nice to ask old people about age (even woman).
Disclaimer:
I am not qualified Mandarin Chinese Teacher or native Chinese speaker. It means my notes about Mandarin Chinese may contain mistakes. If you spot any error, let me know in comments below.
Dominik Symonowicz 