If my 20 year old niece told me she was going to try and have a baby with her boyfriend I'd encourage her to wait and get a stable job/home first and finish uni/work in a job in for awhile first. She doesn't have the maturity needed to raise a child. That's really not unreasonable.
If you are happy being a young mum and keep insisting that you haven't missed out on anything then don't worry what others think. If you're happy in your own life then don't let opinions of others bring you down. You'll cop criticism from all angles when you are a mother.
These days it quite normal that women are encouraged to study/ have a career before they have children so if things go awry they have something to fall back on and aren't dependent on the man/ partner for financial support. It's not for everyone but it does have advantages.
Great mums come in all ages it really doesn't matter how old you are. If you are planning on kids though so not an accident, I tend to think its better to wait till you have a stable home to bring baby into and an income to support the family so there isn't too much financial stress.