I'm with QMI - I'm technically in the group but don't relate to a lot of things that are considered to be their hallmarks. I was always the youngest of every group I was a part of until very recently, so I think I grew up more as a young GenXer would've.
Aside from the culture that lends itself more to sites like reddit and facebook, I wonder if the lack of a strong millennial presence here might also be down to the fact that JWs in their early 20s seem to take the whole thing a lot less seriously than folks did in my day. The less seriously they take the cult while their in it, the less painful it'll be when they wake up to the lies. If they always regarded it as a social club and just slowly and naturally fade from it as they get older and develop normal friends outside the cult, it might not even occur to them to seek out an exJW group online when they realize they were raised in a cult.
That's even assuming that they're having the sort of waking up moment that most of us here relate to - for those that don't really take it seriously, maybe they just let it go and move on and just don't care enough about it to force it into a particular box of being "true" or "lies" instead they just put it in the box of "who cares" and live their lives. The obsessive disproving of doctrine and unraveling of all the deceptions and history of the cult may, in some ways, be somewhat specific to the older crowd that took things more seriously while they were in the cult.