Yeah, this is a bit of a skewed sample here, given that these lists are mainly comprised of Kenyan and Ethiopian men and women, who have historically peaked at much younger ages (with a few exceptions of course).
Add to that, the reported ages of many athletes from both countries are frequently much younger than their actual ages.
If you were trying to ascertain an answer to the question, "at what age does a person's performance peak in the marathon?", you probably also don't want it to be clouded by other factors, such as:
Economics - Lots of Kenyan and Ethiopians will stay in the sport until they've earned enough money to buy property or invest in other businesses, which ultimately results in shorter careers.
Physiotherapy (or lack of) - Access to Injury prevention programs and rehabilitation also prevents many Kenyan and Ethiopians from having prolonged careers.
Societal Pressures - It's much more difficult to ride the wave of a long career with the ever-present threat of extreme poverty. In the instance of a form slump or a bout of injury, the pressure to quit running and to earn some other form of income (usually a manual labour job) is much stronger if the other option means you (or your family) don't eat.
If people want to learn anything about their own likely peak, they should probably compare themselves to similar demographics. The average age of the fastest 100 American performances might be a better indicator of peak performance, as Americans are much closer to having the above mentioned barriers removed.
