coralite,
I do know of a guy that has a queen angel for 13 or 14 yrs. Another guy, M.Dandaneau (whom is very active in te FO forum) has kept a queen for more than that, in a 300 gal system, and his didn't reach 18" either. I belive his got to a little over a foot - as did the other guy. The first guy I mention still has his.
I still don't think a average angel species will reach its full size in captivity - for two reasons. First, most angels do not adapt 100% to the diet we feed them, hence they grow slower than they would in the wild. I think if you compare a 5 yrs old captive emperor, to a 5 yr old wild specimen, the wild specimen will probably be near on full size, but the captive one will probably be 8 to 10" max.
French angels will actually attain, or at least get close to their nature adult size - where as most other angels, under average circumstances, will not
Secondly, as you said, how many people actually manage to keep these fish for 20 yrs? In the care of an average aquarist, in an average system, then my guess is not very many, despite out best efforts. Hence, by your arguement, if they do not live long enough, they will not get to full size. Hence, by that arguement, the biggest that they'll get is most likely be less than 12". I'd agree that if you put an emporer angel in a 180, from the outset, and feed a good diet, and have 200 or 300 lbs of really great LR, have a great system, do plenty of regular good quality water changes, keep no other large fish (i.e. only chromis and gobies etc), then I can see this fish growing very quickly, and living very long, and probably outgrowing the 180 and needing a 240. But in reality, who does? In reality, hom many people keep angels like that? Most folks will throw in a couple of tangs, have a bit less LR, not do enough water chages, not feed enough good food, and in general, have a pretty average system.
Hence, I'll stand by my arguement that most angels under average circumstances will not achieve anywhere near their natural size - simply because we are no where near as good as mother nature when it comes to caring for these fish. Its just that frenches seem to be different, probably more adaptable - they live longer, grow faster, and get bigger than any other angel in captivity.
Regards,
Matt