A teenage girl giving birth to a Down syndrome baby? I've literally never heard of it happening. It's most typically women in their mid-forties. The standard theory as to why this is the case is that spontaneous abortion doesn't occur as readily late in life. In effect, the body figures this might be the last chance, and isn't as picky over which xygotes it allows to develop.
Actually the vast majority of down’s babies are born to women under the age of 35. Being 40+ does tremendously increase the risk of giving birth to one, but because so few women over age 40 and age 35 have children, the down’s babies are still concentrated among women under age 35.