80% of babies born with Down Syndrome are born to mothers under 35 years of age because this age range gives birth most frequently. 

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. 