Isn't it funny how things come around in circles.
In the 70s it wasn't at all un-common to be a young parent(s), my parents were married and had their first child at 19 and 20 respectively. When Mum did the pre school and school runs, she was the 'norm', it was the 'older' (as in over 25) Mums that weren't the norm. When I was in high school it was odd for me to see friends parents who were in their 50s and 60s, (like most these days).
I was travelling overseas when my Mum had her 40th birthday and sent her a telegram to wish her well. I will be 40 in a few years and my kids will be all under 8 by then. So it's funny that what was once 'normal' and not as open to judgement as it is now, is now a bone of contention for some people and it shouldnt be.
If people are going to judge, they should judge the 'behaviour' not the 'person' or the 'age', it's definitely an individual thing, some people are awesome as parents, others not, that goes for any age.
As what is considered an 'older Mum' (I'm in my late 30s), I can honestly say that I have never looked down upon nor even considered looking down upoon or making judgements on younger parents, it's just never occurred to me, as I just don't see the 'age' thing, I am more likely to see the behaviour and in any case its not my place to say anyway. I had awesome young parents and loved growing up with them and am always grateful they sacrificed their younger years to raising us kids.