That really is a hard question. I can only tell you my experience; I had my first baby at the age of 21, I had married my high school boyfriend. The marriage didn't last, just like everyone had warned me. So when we divorced I was 23 and really single for the first time as an adult (I started dating my ex at 15)
At 23, I was a single Mom with a 2 yr. old. Let's just say he spent a lot of time with my Mom. I had a blast! But felt a lot of guilt over my son. The ex had disappeared pretty quickly because he didn't want to pay child support.
I married again at 27 and 3 years later we had another child. at 30, I was a perfect Mother for both my kids. 4 years later we had another (and final) one. It"s been wonderful.
To have a child, you need to be old enough or prepared enough to give up almost all the things you thought you needed. Like sleep, relaxation, going out much and depending on your budget a whole lot of material stuff. Like a wardrobe.
To sum up, If I had a daughter, I would hope she would never have a child before the age of 25, and to be damn sure she had the right husband.
A lot of it really does depend on your income.