At ages 2, 3, and 5, I think we can safely assume none of them would finish.
Given the healthier environment for children in African countries, I would give the advantage to Walker, Coe, Centrowicz, and Ingebrigsten at ages 7, 11, and 13.
Ages 17, 19, 23, 29, and 31 I have to give it to El Guerrouj; he is the WR holder after all.
I personally don't know of any who continued to compete as a Master runner so ages 37, 41, 43, and 47 are a real toss-up. We can probably toss out Centrowicz because he will probably get heavy like his father and most Americans by his 40s.
Once we hit 53, 59, 61, 67, 71 and 73, well, I must go with Coe. Like him or not as head of WA, the guy still looks in great shape for his age.
Beyond that, attrition will start to eliminate the contenders. I'm sure several will make it to 79 and 83, but, just like as toddlers, most will not be able to finish.
By 89, 97, and 101, we can safely say that there will be no finishers and very few will be alive.