Our poly-resins were indeed the best resins EVER used for boat building. I tried to find out about the aging polyester/fiberglass anamoly and talked about it here somewhere. The Navy did some tests on old launches and barges once, I think it was in the 80's, and they found nothing happening. But I remember thinking that the testing may not have been smart enough. IMCO.

A sharp chisel forced sideways into the laminate has a disconcerting way of peeling layers of mat and roving. The stuff is harder than hell if you try to bang strate thru it. But sliceing in...TOO easy. Whether that is age and brittleness, who knows. As you point out, no engineered tests have ever been done on the strength or brittleness of the plastic of these frp boats of ours from the 60's.

Dave intimates that that the A/C hull is thicker in the middle than at the sheer or in the keel area. 338 is approximately 1/8" and 3/8". The 'cold' joint is approximately 1/4". Haven't gone into the mid section, but would be really surprised if the hull was any thicker in the turn of the bilge. The stem was about 5/16s (horrorfied, it has since been considerably beefed up!)
The transom, which has been carved into on 338 is about 5/16s in the flat and thicker in the corners.

Wouldn't you guess that even the casual paint jobs, washing, cleaning, waxing even, have helped to preserve the plastic? The gelcoat could be the major factor in the perservation of the A/Cs. The deck on 338 has no deterioration under the gelcoat I can see. The laminate is just as green and fresh as the day it left the factory.