I created the following thread on TMDb's Seven Up! (a.k.a. 7 Up and 7 Up!) board. Of course that's the 1964 documentary that 56 Up was the most recent every-7-years follow-up to:

https://www.themoviedb.org/movie/51863-7-up/discuss/5a3330390e0a264cc81b64ee

Hopefully a new 7-years follow-up documentary will be produced and will debut in 2019.

Incidentally, the participants were born circa 1956, so would now be 61-62. (That's 1-2 years older than I had thought.) Hard to believe this continuing documentary series has been going on since they were 7!

I saw on Wikipedia that one of the participants, Lynn, died in 2013. Apparently she's the only "kid" who so far has passed.