I think under 21 is young, i think you really need to enjoy your 18th and 21st birthday before getting into the stress of being responsible for a human life. i know a few people who haven't done this and the are now regretting not enjoying partying with thier friends before having kids. 1 who has split from her fiance because they were too young when they commited to each other and thier little girl.
Everyone is different and that is not always the case i always wanted to have kids at 20 (and was ready)but as time went by other things happened and at 27 i will have my 1st. it also depends if you have been with a partner for 5 years then you would feel ready at a young age, and also the people around you and how mature you are. i think 25 is ideal.