shepherdless firstly are you already aware that there have been two academic articles that discuss precisely the point about JWs' prospects for future growth? In the 1990s Rodney Stark wrote an article about why JWs "grow so fast" compared with other churches and he projected JW figures for the next 100 years or so on the basis of either 4% or 2% annual growth, producing a range of 27 million to 194 million by 2090. Both of those look very optimistic now.
http://www.oocities.org/rogueactivex/JWGrow-O.pdf
Then in James Beckford's festschrift the secularisation theorist David Voas wrote a really interesting article arguing that JWs may suffer a dramatic decline in membership in the near to mid term. He argued that once decline begins to set in within a sectarian movement it tends to proceed rapidly, likening it to declines in fish stocks which tend to suffer collapses in numbers rather than gentle declines. I made a thread about it at the time.
http://www.jehovahs-witness.com/topic/172862/british-sociologist-predicts-possible-collapse-jehovahs-witness-numbers?page=1
I think you are absolutely correct to focus on the age profile of adherents as an indicator of decline. Increasing age of membership is a well studied phenomenon in the UK in relation to declining churches such as Methodists, Anglicans and Presbyterians. Peter Brierley has collected data on church attendance, including age profiles and lots of other details for decades.
https://www.ministrytoday.org.uk/magazine/issues/43/352/
http://www.amazon.co.uk/Pulling-Out-The-Nosedive-Contemporary/dp/1853211680
David Voas, Steve Bruce and others have argued that secularisation proceeds as each succeeding generation rejects the faith of their parents. I reckon this is true for JWs as for other groups, they are just a little behind the curve. David Voas explains this sort of cohort analysis and what it means for church decline in this video.
http://youtu.be/YtAR_OGzlcg
It's been years since I attended meetings now so I can't give accurate local information. I can only say that from talking to people who still attend they remark on the total lack of young people. As far as I know there are only 2 teenagers in the congregation, no children, two couples under 40, a few single sisters under 40, and the rest over 40, with probably about half aged 57 or over.