Hmm, let me say it again. Some riders in the past have started their career in the top league (not in a lower league) and become very successful. Age is not proportional to success/ability and there is so much evidence of this I don't think you need me to give examples.
Many of the riders who have started brightly but then stunted/faded is nothing new and to be expected as when the pressure/requirement increases, there will of course be those who don't make it.
The point I am making and which several people, including promoters do not seem to get, is that restricting opportunity by age is totally flawed. Parnytski was under 21 world champion but pants in the grand prix the following year. Crump was also world u21 champ and then won a GP the following season. Age is irrelevant.
What they should be looking at, if they want to develop, is perhaps (and I haven't done a lot of thinking on this) is count experience ( a bit like flying hours for a pilot) and compare that with success, perhaps using averages as a measure, as well as individual successes. Then, a rider could be 25 and just starting out but get a place at reserve because of their lack of experience. A rider who started in one of those reserve spots aged 16 and now 24, still averaging 2.5 points per meeting, should not be ahead of the 25 year old in the queue for a place.