Ultra-marathons and ultra-endurance events are very much less of a young person sport, and we are still trying to figure out exactly why performance can keep increasing into the 40s and even early 50s. Marathoning is a considerably younger peak though, usually thought to be 20-35, so depends on whether you think that is "young" or not :)
Look at the following table that has been compiled about how much contribution from the body's aerobic and anaerobic systems are used for various race distances: