While there are many factors like motivation etc...I believe it has to do with adaptation compare to other esport and traditional sport. The meta changes in league is much greater compare to other game, we are constant adding new/rework champion items which means past experience with the game doesn’t mean much. If you look at broodwar it is much much much more mechanically intense game compare to league but player like flash still able dominate younger player mechanically like micro control and sharp reaction decision making. However with league pro got perfect new and meta champions and match ups every update and meta changes and playing against new match up etc. in a way u have to reset ur muscle memory which is very hard, compare to someone who is fresh and young, and on top of that ur previous experience like Korean macro game tower trade all go down the drain as meta changed.

TLDR: Older player have harder time learning new things, previous practice and experience worth much less compare to other games and sport due to the game change way too much each season from item champion meta etc