That is actually a very good question.
I don't know the right answer myself, I am of the older age group I would say - I wasn't around during the DOSMugen era but I knew about it when I first started using mugen, which was probably some time around 2005 maybe.
At that time I was in the middle of college, and i am in my 30s now.
I think that there are still quite alot of the people in my age group still active in mugen because the type of content that we like is based on the pop culture of video games that we were most interested in at the time we were young.
Games like Street fighter III guilty gear and melty blood AC and of course Capcom vs snk 2 which was a hugely important game to my generation in the early 2000s.