It's not so much about your body being prepared, as it is about you being emotionally ready. Kids can really yank your chain, much as you love them. Knowing what I know now and looking back, for me the right age would have been 25 through 35, if it had been medically feasible. (It didn't happen for me then due to an appendicile abscess that removed my ability to conceive naturally. It took IVF and lots of tries, all of which came later.) I am a very patient mommy now, but older. I wouldn't have been a very patient mommy if I had gotten pregnant younger than 25. And patience and the ability to keep on moving forward are really a lot of what it is to be a parent.
I agree with Annie. This decision shouldn't rest on when you are most physically able to become pregnant and bounce back after a pregnancy.
This decision should be made with MUCH greater importance being placed not on the pregnancy, but on the ability to care for a growing child.
So, the best age is when you're married to a very supportive and loving husband who can provide a home, and food, and a lifestyle that will make you comfortable. You don't have to be rich, but you do have to be able to afford safe housing in a safe neighborhood with good schools, good food, reliable transportation, etc.
The pregnancy is just the very very beginning of the most important commitment you will ever make in your life - to be a parent.