This is really going to depend on the person. I don't believe in attaching ages to these things, I'll just do them when it feels right. I think what's far more important than a number is having all the life experiences you want to have before settling down and having kids (like traveling, dating different people, getting your education etc.) That said I think any time in your teens is too young to be serious and settle down, because generally you're not an adult and won't have had that life experience.