No, your teacher was wrong.
It must be either "May I know how old you are?" or "How old are you?"
Student or Learner
"May I know how old are you/you are?"
When I was at school, my teacher said the correct one was "May I know how old are you" but I don't think so. Was my teacher right?
I'm sorry. I clicked "Go back" and accidentally posted the same thread once again.