no particular age, but when you can support a child, yourself and are in a stable relationship with the coparent or at least the people in your life. if you're still calling yourself a girl you're probably not old enough
I think that when you feel ready. However, some girls "feel ready" at age 17, which is a bad idea. I think you should go to college first (if you are planning to), and wait until you have enough money to raise a child!