Several years.

This new arrangement only applies to each country's top leagues!? Therefore riders could still ride in a Championship team without being affected by the new rules? Or is this open to interpretation...

Please elaborate 

Age is immaterial.  For example. they may get a place because they're 20 but end the season with a 2.5 point average and not get a place the following year as no longer u21.  It's not about development at all, just outright ageism.

It hasn't worked in some areas of football where a human studies the evidence provided by technology and still make the wrong decision but goal  line technology has worked 

So maybe the Poland + 1 league was not happening after all?  This is good for the UK and Denmark, if riders remain unhappy with the Swedish tyre rules many may gravitate to UK + Denmark.  U21 rule I also like it will force teams to develop riders. It also will bring 7 likely completely new international U21 riders to the UK which may help widen the gap between Premership and Championship as I don't see those riders doubling up. Link for those interested: https://ekstraliga.pl/en/se/news/new-rules-for-riders-in-speedway-leagues-from-2027