I haven't been in about a year, so I may be a little off, but I'd estimate the median age of my former congregation to be about 45. That may be a little conservative, though - it may be closer to 50. I think there's roughly 90-100 publishers assigned to the congregation with an average sunday attendance around 50-70. This is in the southern US.
I like this approach to estimating the decline. It will at least give us a reasonable estimate on the upper limit of how things will trend for the cult. I definitely agree that it will be a mostly slow decline with no mass exodus (unless something really big happens) and the decline will be mostly driven by the old ones dying off over the next decade or two. The problem, though, is that JWs aren't even keeping up with population growth rates so the decline will be more than what you'd estimate using normal models. To get more accurate you'd have to adjust the life expectancy figure to account for the young ones that leave, but that would be a tough thing to estimate.