Average age of a successful BG completer is 37 1/2. The most common age, fittingly, is 42. I was 46 when I did mine. The annual summary shows age spread, I post it here and on the Club's website.

I checked on the average finishing time per month and surprisingly there isn't much variation, between 22h40m and 23h40m but nothing showing that midsummer times are statistically faster than midwinter ones though of course winter Rounds are somewhat self-selecting in that most (all?) who attempt them have something significant in reserve.