North America here.
When I joined this site, under different name back then, in June 2000 it was largely Americans. However, very early on there were a lot of people from Europe joining in and it looked to be evenly distributed, about 50/50 from I can guess.
These numbers don't reflect a lot of people from Asia and Australia yet I can come up with several off the top of my head. Anyway, MR has become quite global.
What also started largely as a teens/early 20s average age has become somewhat older probably averaging now in the late-20s by now. Many of the young people who would be that old by now have long left the site. As far as I know, from earliest days, Kainjow the moderator and myself are only original summer of 2000 members still here.