When you are ready, and by that I mean when you both are mature and financially stable, emotionally stable, and are together. Yes, she said it! TOGETHER! Children deserve to have 2 parents. You may not have had 2, that's fine & dandy, but there were times you probably wanted, needed 2. Give your children the best start in life that you can.
It is hard enough to raise children under the best circumstances, let alone when you all have problems. Like no job, no money, no place to live, no partner, etc. Too young, you are not ready yet, too old & you'll be in the nursing home when they graduate college. Plan it out, for goodness sake.
People plan out what they are doing for prom longer than when & how they are going to have a baby. So, think of others, think of your baby. Plan it out.