Here is the Pew Research source:
http://www.pewresearch.org/fact-tank/2016/07/11/which-u-s-religious-groups-are-oldest-and-youngest/
Someone else has posted it a few days ago and commented on it.
Yes, the reddit article is well written. However, I think the author of the article misunderstood the statistics. The author appears to be comparing Jehovahs Witnesses to a bunch of "mainline" protestant religions, noted that those protestant religions had greater proportions of over 65s, and drew the conclusion that over 65s are leaving Jehovahs Witnesses in droves.
Looking more closely at the data and the original 2014 Pew Report, what is actually happening is that protestant religions (particularly what Pew defines as "mainline") are dying rapidly. Because the chart contains lots of smaller protestant religions, and (for example) only one line for Catholic, it creates a skewed initial impression.
The best way to look at the chart is to compare JW to the "All US adults" (shown two thirds the way down). Its the adults under 29 that are missing, not the over 65s.