The first time I got pregnant, I was a comparatively young mother, for my demographic: I was 25, in medical school, surrounded by classmates who, for the most part, were not reproducing yet. By the third pregnancy, 11 years later, I was over 35, which classified me, in the obstetric terminology I had learned in medical school, as an "elderly multigravida," that is, someone who was having a child but not her first child, after 35. (If it was your first child, you were an "elderly primigravida," or "elderly primip" for short — even as a medical student, I had a strong sense that no woman had invented this terminology.)
So by certain standards, I have experience as both a somewhat younger mother and a somewhat older mother, though not at the extremes in either direction.
National Vital Statistics Reports data released in January showed that in the United States, birthrates shifted in 2015: The birthrate for teenagers dropped to 22.3 births per 1,000 females ages 15 to 19 that year, a record low for the nation. And for women 30 through 44, the birthrates were the highest they have been since the baby boom era of the 1960s.
And as birthrates shift toward somewhat older mothers, researchers continue to look at what that says, both about who is getting pregnant when, and how that is associated with how their children do, especially when it comes to cognitive outcomes. (There's also been some interesting research recently on paternal age, but these studies focused on the mothers.)