You are required to have a bachelor’s degree in any field. However old you need to be to have acquired that bachelor’s will be sufficient. You also typically need to be a native English speaker, and it helps if you are from a majority white country and if you yourself are white (that’s terrible, but it is what it is. I did know a couple of English teachers of Indian descent in Japan while I was teaching there, so it’s not all white people, just mostly).

Any age. Check out new posts on /m/japanteaching about this.