Children of youngest and oldest mothers at increased risk of developmental vulnerabilities. Children born to teenage mothers have the highest risk of developmental vulnerabilities at age 5, largely due to social and economic disadvantage, a new study of almost 100,000 school children has found. The risk declines steadily with every additional year of a mother's age up to 30 years, then increases slightly after 35 years and older -- to a level similar to the risk for children born to mothers in their early 20s. Source 1e.
With people being different and everyone who wants to have children able to do so at a time of their choosing I do not think that one can really say when it the best time. Some people want children when they are in their late teens to early twenties and others wish to wait till they are in their late twenties to early thirties. I guess while science and research will say mid twenties to mid thirties I still think it is up to the person and how they feel it best for them.
I think your early twenties is a good time but only if you feel you are mature enough. I would only have children if other things were secure: money, home, car, etc. Of course I would want t good father at home, I'm not game to be a single parent.