The whole study is interesting, but that's only one angle, by religion.
It does seem the Christian religions are having a harder time as they have fewer young ones than atheists, Hindus, Judaism, Islam.
I look at the Hindu one and I'm probably making some assumptions. Most Hindus are Indian. Their number significantly drops off the older they get. Is this because of the HUGE number of Indian contractors over here? I worked for a Fortune 500 company and there are plenty of major headquarters where I live. So I've worked with contractors and have seen a huge spike of Indians in the area. The contracting firms tend to send younger people as it tends to make them look youthful and full of energy. But they don't send too many older contractors. So that may explain the skew towards the youth. Because many of the Hindus are over here as contractors and the young ones get sent.
I think Balaamsass2 is right. Spanish speaking congregations are propping up the JWs. I have a family of Mexican JWs living right next door to me. The couple is in probably their late 30/s, the husband maybe early 40's. Her brother's in his 30's. The couple has 2 young kids. My son's best friend is a JW from another Spanish speaking congregation.
Could it also be that the older you get as a JW, you die off? How many people forsake college and good jobs and then when they're older, they can't cope with health care issues and spiral down and don't live as long?