Actually beyond that it's a pretty awful study. The graph is showing rates levelling off for all groups. It makes sense for people that are old to have been divorced. The actual rate of divorce for people that are 18-34 right now isn't going to be known until they are 55-70. The data is already showing that they will have a lower rate that today's 55-70, but again, to blame the divorce rate drop on the 18-34 age range when it's across the board, is kinda ridiculous. If you look at the graph of the 18-34 chart going back to 1980 it's pretty much bouncing up and down from 11-13 percent consistently.
An 8% change in any behavior on a sample size as large as the country is huge.