Bad stuff goes in, bad stuff comes out.
FWIW, we definitely had ADHD kids when I was a kid in the early 70's. I remember my 4th grade teacher actually taping a boy in our class into his desk because he couldn't stay seated. It's always been around, just not given a name. Kids were just labeled "bad" or "dumb" or (my personal favorite) "socially retarded."

The pace of our society, lack of appropriate levels of exercise and poor diet have all contributed greatly to making subsyndromal kids pass the threshold for diagnosis.
I believe ADHD is over-diagnosed, over-medicated, and that a good number of kids could be appropriately managed with behavioral interventions, discipline and a purging of sugar and chemicals from the diet. Also, it was in the 60's and 70's that the food industry started in earnest manipulating food content. I don't think that's a coincidence.