Answer:
The statement below can be addressed as "formal".
"What birth order are you?"
This question has been used in psychology "check in sheets" when obtaining client information.
You also have some non-formal ones which are:
"Which one are you among your brothers and sisters?"
"Which one are you among your siblings?"
You can also say "Are you the youngest in your family", to which you would reply. Yes, no, and then provide the appropriate information addressing your "nth position".
