Aged 47 now. Started at age 42 with a 5 mile race that I've done annually ever since and every race has been 36 minutes and X number of seconds (haven't kept a record,so not sure which year was my PB but I don't think there has been much improvement or decline - this year though was 36:20).
Done my first two 10k races this year (both off road) and did them in 46 min 52 sec and 45 min 3 sec, so still improving there!
Also done 2 Tough Mudders (13.5 miles) this year but as part of a team, where there was a lot of waiting / dragging slower members round, so time was not an issue. Obstacles were fun though.
Before I ran just occasionally, as cross-training for other sports but at the moment I think it has just about risen to the top of my personal rankings for enjoyment - and the amount of time I am devoting to it.