Most of the times I am good at guessing other people's age. With Kipchoge I was really confused! His voice, his face his move. He looks older than me (37) and now it's probably getting clear. But still I am not surprised. Maybe you get older and you loose some aerobic capacity but your are getting more experienced and the body more immune to injuries!

That was my initial reaction, too. Or is long distance running much less of a "young person sport" than others?

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:

From https://www.runnersworld.com/advanced/a20805305/owners-manua...

  Distance    Gastin Aerobic/Anaerobic Ratio
  Marathon    97.5/2.5
       10K    90/10
        5K    84/16
  1500m/mile  84/16
      800m    66/34
      400m    43/57

Marathon running is mostly aerobic vs running 1500m or even shorter distances like 800m or 400m.

So any loss of speed as marathoners get older won't be catastrophic versus competing in shorter races.