Here's a question for anyone listening that occured to me this week :
Yoda trained Jedi for 800 years, and was 900 when he died. At the time of Revenge of the Sith he was physically as strong as ever ("Size matters not" etc), obviously battered by his confrontation with Palpatine but in one piece. His pride was in tatters and his confidence was low, but essentially he was alright.
(As far as we know) he didn't leave Dagobah in those 22 years between Sith and Empire, so apart from possible contact with Obi Wan he would have been alone and, importantly, not training Jedi.
In the year between Empire and Jedi Yoda deteriorated rapidly into a frail little man. So, my question to anyone reading is - was it the exertion of training Luke Skywalker that killed Yoda? Or are other factors at play here?