Interesting story here based on an anthropological field study in Quebec. The people behind the study would certainly be fun to study. A decrease in age at first child from 26 to 22 wouldn't be fertility per se. Human females have always been fertile much younger than 26. One could ascribe it to social mores. That's not entirely convincing, though, because teenage mothers were fairly common before the 20th century. My other thought is that for some reason miscarriages used to happen more often. So many questions.
Overall I do believe the implications of the study. In a way, microevolution is just an extension of the little mutations that always occur between parents and children. Plus ça change and all that.