I fully expect that this is the ushering in of a new era of athletes reaching national and world class times at ages 16-19.
I believe this is due in large part to the depth of specific teams and training groups that allow athletes to chase after other world-class peers.
Kessler -> Willis + Ferlic
Newbury Park -> Nico
Jakob -> Middle and Elderbrigtsen
Hocker/Teare -> each other + Hunter
In all of scenarios, we have seen athletes come in and just do a percentage of the workouts that the established athlete can perform and get very similar results.
For example, Kessler may only do 6 x 400m while Willis does 10. Or Lex Young may do 3 x mile while Nico does 4.
The key is not that the up-and-comer slows down the pace he just does 60-80% of the workout at the same pace.
If you are a coach of an athlete that wants to run sub-4:00, but doesn't have a sub-4:00 talent already on the team, then you must look at the workouts that a sub-4:00 runner would do and have your athlete do a scaled down version of it. Having them do all of their running at only their current fitness is not the way to go.