I'm not sure that "median age" will be particularly informative in estimating the potential decline of the WT. I think a much better source would be to look at how quickly the growth rate has declined over the past fifteen years and project that continued reduction into the future.
http://www.jwfacts.com/watchtower/statistics.php
It's also worth noting that the decline isn't just in the lower recruitment. It's also in the fact that what qualifies as a "publisher" has been downgraded to someone who reports as little as fifteen minutes per month. Many of the publishers are JWs purely for social reasons and have little understanding or interest in doctrinal matter - and even less interest in donating funds or performing parts.
Pioneer hours have been reduced. Aux pioneer hours have been reduced. Length of talks have been reduced. Meetings have been reduced. Reading material has been reduced. Studies have been grossly simplified. Baptism is being encouraged younger and younger. Etc.
The WT has been fudging the numbers for some time. And there's not much more they can do at this point to keep cooking the books. The WT already is in decline. It has been for almost a decade now. The real question is how much longer can they hide that fact?