Jim I struggled to get my kids 19 & 21 covered on when I first tried on my 5 speed 3 wheeler none of the Morgan specialists would insure them until they were over 25 The same also applied to the +4 we had for a year or so. We did manage to get them covered on Admiral to drive the +4 though.
The only way I could get around it was to bundle our vehicles together in policy with Chubb. That's got any driver cover on all the vehicles and the kids are named as drivers for the 3 wheeler and my Datsun. They can't drive the aero but ironically my sons girlfriend can so that makes him