Wow 18 is too young. A study shows we don't reach adulthood until 30. At 18, there are still a lot of changes in your brain. That's going to be too hard and maybe even damaging for the woman (or man if he's at that age)

When the person is mature and ready for the marriage. Someone can be 40 and still not mentally ready for the responsibilities it entails.

No one is born for marriage, so keeping single is an alternative for those who do not think marriage is suitable for them. However, in terms of birth, the age for a woman would be subject to around 20-40, though women might give birth after 40. 

I think that, if you love each other, then the sooner, the better. Ok, that might sound debatable but I have my reasons for saying that. My main point is that you shouldn't pull back (postpone) this step deliberately. You are stronger together, your opportunities are wider when you're a family.
I wish the society  wouldn't frown at young people who get married at age 18-20-ish. In the end, why is a career more important than family?! It's days with your loved ones and not work that you will always remember.

I wish we would have married sooner and I wish we would have had our daughter sooner. Only when you have it, you realise how significant and good it is.
And I wish, everyone had a positive experience.  Maybe then there wouldn't be a fear of having an unhappy relationship or ending up in a divorce.