I think maybe easier access to healthcare, mental and physical, could help?
Also a lot of this might be signs of an overworked populace? (How many jobs are people working to make ends meet? Do the ends meet after all the jobs are worked? etc). So we should probably look at that as well.
But I think one of the biggest things we should take a long hard look at is our lifestyle. (I know that work hours are a part of that, but I broke that out on its own because it's at least a bit easier). What are we doing during our days? (A lot more screen time these days than there were pre 1999. How many hours do people spend checking reddit/facebook/TinderInstaChat? What impact does that have on physical and mental health?)
What are we eating and drinking? (This might have something to do with it?)
Are there any environmental factors that have arisen as a result in subtle lifestyle changes during the indicated time period?
Etc etc etc.
I guess, in short, I think a comprehensive trend such as the one outlined in the study likely requires a comprehensive look at possible causes.