Which is proper to use: 'How old are you?' OR 'How young are you?'

I was a bit intrigue with these two questions that are sometimes used by people of different social classes. I appreciate any comments or opinions as regards this matter.

They are not used by people of different social classes. Every native speaker uses the first one.

Really?
I’ve heard people say how young are you to me lots of times when I was younger xD

It’s really not socially acceptable to ask either question unless there is an absolute need to know.