I am replying without reading everyones posts so sorry if I may step on toes!

I think our bodies may have fertile 'years' because God assumed we would be mature by this age. Sadly, (and let's be honest I know there are many good 'youths' out there but majority rules) teenagers and 'young' people are getting more selfish, reckless, violent, abusive (of other people and their own bodies with drugs etc) every day and therefore that is when we need to consider if bringing a baby into this world by those particularly reckless people is simply... sad and wrong. Therefore, it is fine to consider 'how young is too young' but lets not forget society is unfortunately 'misbehaving' younger and younger. Hope that makes sense...