For me, age 20 would have been way too young to have a child. I was still in college, had no idea what I wanted to do for a living, and still mostly depended on financial support from my parents. If I think back on what made me think I was ready to have children, I'd be looking at age 26. At that time I was in a loving relationship, finally doing a job I enjoyed, was making a decent salary with good health benefits, and almost had my car paid off. At that time, I was ready, but my wife, who was 24 at the time, was not. She was still in college and wanted some more time to enjoy life outside school. So here we are 3 years later, both agreeing that now is the perfect time.
I don't want to discourage you, and there are tons of happy young moms on MDC, but I would wait. When me and my wife starting discussing my yearning to have kids, she helped me realize that just because we could try didn't mean we had to. Babylust is incredibly powerful, but children totally change your life. I would take some time to talk to doctors about your fertility (I know that is a concern for you), and do some thinking about what you think your day-to-day life will look like with a child in your care. Maybe also talk to some younger single moms to see what their lives are like.
Good luck with whatever decision you make!