I think to some degree it's about better public and medical education about recognition and reporting of ADD/ADHD. I have seen excellent best-selling books on the subject in the last 15 yrs where almost none existed prior to that. All these talk shows and online websites make it a cottage industry almost. Doctors never could report what they weren't really aware of...20 yrs ago.
I think it's very hard to determine whether or not there any more (or less) incidences of ADD/ADHD. I think that this generations grossly shortened level of concentration and attention timeframes have been adversely affected by things like MTV and other media. Those constant interruptions from devices like TV, smartphones, computers, etc...play havoc with focus. Participation with video games have taken their toll, too.
If you want an amazing comparison of concentration spans of audiences, look at the pace of a show like Father Knows Best or Leave It to Beaver and compare the relative pacing of a sitcom like Big Bang Theory or 2-1/2 Men . The shows from the '50s had scene changes about once every minute. The new shows change scenes about ever 5-10 secs.
Then there's the amount of TV and computer watching going on in the last 20 yrs. Mercy...need this behavior continue unabated?
I contend that and all the other effects take their toll on young children's concentration spans.
Then there's that massive effect from sugar and caffeine increase in diets.