In addition, and I'm not trying to pile on or anything, just pointing out how much stuff has changed, I think Nancy Drew and the Hardy Boys would've been YA(I'm not sure if the same categories were the same but I mean thought to be for what we consider a YA audience), no? I remember reading some when I was quite young; my grandparents had a shelf full, and weren't all of them like, 16 or 18 or something? I think the boys were older than Nancy Drew but I'm fairly sure they were all full-on teenagers, and I swear a cover of one of the Nancy Drews has her driving a car.
I mean I think they're tame (from what I recall), and I did read them when I was, I have no idea, probably somewhere around seven or eight, but I think they weren't meant to be for younger kids when they were published.
That series about the brothers in the late 1800s, early 1900s, (they weren't written then, but set then) whose names I can't recall, but who got into all manner of trouble, those I think would be MG. They got the first toilet in town, and their friend got lost in the caves -- one of the boys was the super smart trouble-maker.
Encyclopedia Brown, also old-school MG, yes?
Along with the Judy Blume stuff wasn't there that series about a girl, Alice, I think, who aged up through a bunch of books, from MG to YA, and went through bullying, sex stuff, etc.?