How about this change? The mid-am and senior-am champs become part of each team. That'd still leave 8 spots for the 18-21 set.
There used to be more older guys on both teams. Trip Kuehne was on the 2007 team at Royal County Down, don't remember if he was on the 2005 team at Chicago or earlier.
Some of the older former Walker Cup guys have been grumbling about the lack of older players, or so I heard from a former captain.