Not unlike Autism Spectrum Disorders, I think ADD/ADHD has always been around, it just gets diagnosed more often now. If you don't know what to look for, you aren't going to find it, right? 50 years ago or more, kids with severe autism were labeled as "retarded". Kids that were hyperactive were just labeled as discipline problems and often left or got kicked out of school early.
At age 62, I have just now been diagnosed as ADD (inattentive type). I was smart enough that I compensated when I was in school and still got As and Bs. That was 50 years ago, when ADD was primarily diagnosed in hyperactive boys, not girls lost in their daydreams. It was thought that you grew out of it and that adults DID NOT have it. There is still resistance to diagnosing adults today, with some insurance companies refusing to pay for ADD meds for anyone over 16. In my case, I inherited it from my dad, who was also bipolar (I'm bipolar 2). Of course, he was never diagnosed with ADD because he was born in 1931. But better late than never, right?