I wonder when it stopped because all of the schools I attended in the 70's-80's did ability grouping. They did mix up ages but only for 6th and 7th grade for some reason. With the other grades they kept the ability grouping within the grade. Homeroom teachers of course had kids of all different abilities, but the kids rotated around and went different places depending on the subject. So I was with one group of kids for math and a different group of kids for reading (I was considered advanced for reading but not for math). I never thought negatively about myself for being in a lower math group or the second lowest gym group. I don't think I thought much about it at all. In a way, it was a relief being with kids at a similar level--in gym for instance. Who wants to play sports with kids who are way more advanced?