I agree with BadWolf that this particular policy should be kept the way it is. The rank requirements are designed for a specific age group, and that is who should earn them. Adults have a specific role in the program, in service to the youth, and that is what we should be doing.
But the historical discussion is kind of interesting. One thing I would like to know is whether the handbooks, advancement guidelines etc. of 1910-1952 ever actually said that adults could continue to work on rank advancement, or whether there was simply a lack of any age limit. In other words, it may be that they simply forgot to specify an age when rank advancement must stop - stranger things have been forgotten - and by the time someone said "Hey, why do we have all these adult men working on merit badges and earning ranks designed for teenagers?", it was too late because by then it was sort of an "entrenched" thing and they didn't want to take away an opportunity that some people were relying on. And it took years before National finally bit the bullet and said Ok, this has to stop - and still more years before they got all the councils to make it stop. Unless someone has any actual evidence to the contrary, I think my "We forgot, and then it was too late" theory sounds pretty plausible.
Left for war and could not finish Eagle? Sure.
This actually shouldn't be an issue now because the minimum draft age is, conveniently, the day after all Eagle requirements (except for BOR) must be completed. I am not sure whether the draft age was ever below 18. I am pretty sure it was 18 at the time of WW2 - at which time you could still work on ranks after turning 18. One of the comments on Bryan's blog suggests that this further delayed changing the policy, because if someone was 18 and a half and still working on ranks when they got drafted, when they came back the BSA didn't want to say "tough luck, you missed out on Eagle because you were fighting in a war." My father actually did get a couple of merit badges after turning 18, but I am not sure whether he got any after he returned from the military. (He was drafted about six months after turning 18.) He finished at Star, which he had earned before turning 18.
Died premature before finishing Eagle? Absolutely!
There's an award for that, "Spirit of the Eagle." Families of Scouts who have passed away can apply for it.