How Old Do You Have to Be...?

I hate it when I run in to adults who attended a daycare I worked at when they were children. I was their teacher and now they are grownups. I am taking a class on Wednesday nights. Tonight was the first night. There was a girl there who is a teacher. Yeah, she used to go to the daycare where I worked. I was her teacher, now she is a teacher. How old do you have to be to be a teacher nowadays?