I've just started playing about with SEDE (a very nice tutorial here) and one of the queries I eventually wrote was this one on the votes per month. They had quite a distinct trend I hadn't really expected. Thought someone might find this interesting.
The months are numbered as you would expect (1 being January and that low point at 5 being may).
I know it isn't 'activity' exactly and I might try to change that (take into account number of questions/answers made maybe) but still a measure of something all the same.
After Whoosh's comment I thought I would see how age was distributed across WB accounts. I did that (I think) in this query. Which produced the graph below:
This seems to support that a lot of WB users are under 25 but the bulk are still over. Of course some people have made up their age (unless we have a three year old and 42 97 year olds on here) but I would expect a trend to still show.
I'm going to try to make a query to pick out the votes cast and posts made by each age (it could be the younger users ask or answer more questions or vote more liberally and that leads to the trend we see).