The information in the report appears to be based upon data from the 2014 Pew Report, but further extracted and expanded.
Both the 2014 report and this report states the median adult age of a JW in USA is 50. This is more important that I think people realise. The median age for JW increased dramatically between Pew's 2007 and 2014 reports. History shows that an excellent early indicator of a religion or population in decline is when its median age increases over a certain level. It appears the religion in USA has reached that level.
To put it in context, the reports define adults as those over 18, so "50" is equivalent to a population median age of around 41. In the last few years, the median age in Japan has reached that level, and Japan's population is now in decline.
Although JW appears in around the middle of the chart, you have to remember the 2014 Pew Report showed that most Christian religions were losing members and aging, with "mainline" Protestant religions faring worst. So being in the middle of that chart is not a sign of health.