I think Nijel was young, just not 18. He came from a different situation than the Kenyans, who have to be absolutely stellar to get foreign competitions, so they need to train for years with groups to make it there and are likelier to be age cheats for that reason that it helps to get out. Nijel, however, was probably 20-21 in 2012. He is really an outlier here because he held it together for ten years, running that 1:41.89 so much later and not doing it again just because of an injury in that race. Note that two Kenyans on the list ever ran faster. Brazier did as well and his birth date is well documented. You will almost always get continued improvement from U20 male athletes. The doping, by the way, would not be why others wouldn't continue to improve, because the doping wouldn't likely stop at 20. They essentially have to peak to make foreign competition because the Kenyan comp is so good.