I'm not sure there was "no such thing" as ADHD 20 years ago. It just didn't matter as much. There were ample good jobs in manufacturing and agriculture and other manual trades which required only a marginal high school education, or even less. Kids (especially boys, who are the bulk of diagnosed ADHD cases) who couldn't concentrate on "book learning" simply stopped pursuing it by their late teens, and went into a steady job which neatly dovetailed with the classic ADHD ability to hyperfocus on repetitive physical tasks. But the world has gotten a lot more sophisticated, and unless we want to give up computers, the Internet, high tech medicine, etc., we need a much larger percentage of the population to achieve substantial levels of academic learning.
However, the assertion that something on the order of 20% of US children need to be on medication for ADHD is ludicrous. If there are really that many children who have the symptoms of ADHD, then the schools need to change their programs to better suit ADHD kids (this would tend to mean a return to "rote memorization" and "drills" and similar politically incorrect educational methods, which seemed to work fine for non-ADHD kids too, back in the "good old days").
"It just didn't matter as much. There were ample good jobs in manufacturing and agriculture and other manual trades which required only a marginal high school education, or even less."
Agreed. We need a strong system of apprenticeships in labor-intensive jobs for these kids. Switzerland has this system and it works great.
But we don't want a system for this created by gov't, either, it should be created by private industry. Problem is, unless it's something that the kids get school credit for the school administrators will scream because they lose money if those kids are pulled out of school early.