80% of babies born with Down Syndrome are born to mothers under 35 years of age because this age range gives birth most frequently.
There are quite a few mums on our Positive About Down Syndrome New Parents group who are young, many in their twenties and I can think of more than one who is under 20. There are many mums who are in their thirties and a good few in their forties. From the reading I did in the early days of my pregnancy, I learnt that the chance of having a baby with Down Syndrome doesn't, contrary to popular belief, increase with age. I also remember a conversation with the screening midwife where she confirmed that age was a very small part of working out the 'chance' (I am pretty sure she said risk).
So a plea from me, when you first learn that your baby has a little extra, you may feel guilt that if you hadn't had a baby in your late 30s they would not have had Down Syndrome. Firstly, you are one of the lucky few to have a baby with Down Syndrome, whatever your age, and second, my understanding is that age has nothing to do with it. There is enough mum guilt flying around - don't add things you have NO control over into the pot. Lecture over. Enjoy your baby, however young or old you are.