actually, really long distance endurance events are mostly dominated by older people. look at marathon runners for instance, most marathoners are in their late 30s and 40s. Or Randoneurs who participate in Brevets. Mostly riders in their 40s and 50s. The ultimate Brevet is a 1200km (750 miles) ride, you have 90 hours (3 days 18 hours) to finish. before you enter one of these, you have to complete a 200km, 300km, 400km, & 600km brevet in the same year. I think you'll find VERY few riders UNDER about 35 who could even begin to complete a big brevet.

http://dbcgoldrush.org/
http://boston-montreal-boston.com/
http://www.rmccrides.com/lastchance.htm

for some US brevets