I am currently in the middle of doing a statistical study about running times on all levels over the past 10 years, and have found that the amount of confounding variables surrounding the faster youth is astounding. We could chalk it up to the shoe technology, but the reality is that it's only one facet of many surrounding the explosion of talent over these past few years.
I would wager it's likely this factor along with
1) Increase in shoe technology on a national level.
2) The pandemic allowed for the sport to gain more participants/popularity
3) Which, in turn, allowed for those in the sport, to specialize more and train on a more dedicated level at a younger age.
I'm sure there are more, but that likely is the largest three, along with increases in technology/training/and knowledge availability with the exponential growth of the internet and more advanced development of training.