I was 25 when I had my son, so not young but not old either, and I still get judgement. I don't think it matters, as pp said, if someone is the type to be judgemental, they will find something to critique you on.
That being said, I don't really consider a 20 year old to be a "young mum" in the sense that you are talking about, as in being judged by society. It's on the younger side, but you are an adult, you've finished school, and if having children is your priority over other things 20 year olds do, like university or partying, then go for it.
I see 'young mums' as those between 15 and 18 or so, and I don't judge them, I just worry that they've set themselves up for such a hard time. Finishing school at the same time as raising a child and worrying about finances, a relationship and all those adult things, mostly all by yourself because your friends are still all about school and partying would just be so hard, and I don't think anyone could deny that.