I think I've seen one on here live till 19 (??) but that age nowadays seems to be a rarity (the 12-15 years is probably more accurate). The problem is, as they are wild animals, they still hide illness very well - once the animal exhibits the symptoms and the keeper recognises them it can often be too late. Couple this with (potentially) sub-standard husbandry/housing (especially no UVB provision) and inadequate diet (not the variety or nutrient composition of wild bugs) and you can see why some individuals don't live as long as they could do (not saying the people you know can't look after animals but you get my drift).
In my opinion, I think part of the problem is a decrease in fitness due to the lack of genetic diversity or 'new blood'. It is inevitable that there will have been/continues to be some degree of inbreeding (especially where morphs are concerned) and especially considering there are little/no wild caught (or F1 etc.) individuals available. Inbreeding itself isn't necessarily bad, but over generations it may become a factor.
Again this is my opinion but out of my two AFT's, the morph individual is definitely the weakest. My normal goes from strength to strength whereas the morph has become ill a couple of times and is tiny compared. Obviously I can't be sure; they are from the same breeder, kept in exactly the same way, provided the same food etc, but there is still an obvious difference between the 2. Other AFT owners that I have talked to have also found the amel morph a bit more problematic than normals, obviously it could be a coincidence though.