On February 26, 2012 Ariel "Arik" Sharon will be 84, although it is very unlikely that he will celebrate his birthday. This is because he has been in a coma, probably brain dead as well, since January 6, 2006. He will probably die without ever having regained consciousness.
But imagine, for argument's sake, that tomorrow he were successfully revived and like many elderly people who suffer strokes he would be able to recover cognitive function eventually. This would at minimum probably take a year before he could speak without slurring his words and think and function at a high level. And it might be possible--even likely--that while in a coma he had developed some sort of dementia. It is doubtful that voters or party functionaries would trust someone of that age with the premiership. When Sharon suffered his stroke and was subsequently permanently relieved of the premiership he was 77-78, older than any previous occupier of his office. Previously the oldest premier at retirement had been David Ben-Gurion, the first and third prime minister who retired in June 1963 at age 76.5. So no Israeli prime minister has crossed the 80-year-old barrier and none is likely to ever do so--particularly someone who has suffered a severe stroke. Between Ben-Gurion and Sharon Golda Meir had retired at age 76 in 1974 and Shimon Peres at age 73 in 1996.