Napoleon could have lived into his early 80s, though it was somewhat rarer than today, but it was not completely unheard of. If individuals lived past adolescence, they usually managed to live into their 50s and 60s and even into the 70s. Around one in thirty adult males seemed to make it to 80 in the early XIX century. Though it was mentioned before, with his brothers Joseph and Jerome living till the ages of 76 and 75. George III was born in 1738 and lived until 1820, being 81, so I imagine Napoleon making it to such an age is doable. If he were to live to 1849 to 1851, his image had been somewhat rehabilitated by that period and perhaps he is no longer seen as a threat. Additionally, if the Bourbons are overthrown as they were he might be pardoned by the British and allowed to return to Europe. He was in his 70s as he would be an old man and no longer seen as a threat, though this likely depends on the sort of government in power in France. A liberal revolution might even proclaim a republic, instead of calling Louis-Philippe (who himself lived to the age of 77) to the throne, and perhaps the Bonapartists have another shot.
On a side note, I looked at my own family tree (fortunately Portugal has detailed parish records that go back to the sixteenth century in most parishes) and I looked at the oldest male ancestors in the pre-modern era for which I have records. I have one 8th great-grandfather whom lived between 1684-1772, dying at 87, a fourth-great-grandfather living between 1801 and 1889 to the age of 87 and another fourth-great grandfather living between 1791 and 1883 until the age of 91. It seems that women, if they were able to pass age of 42 when natural child-bearing ends, without death, were more likely to make it to the age of 80.
I wanted to analyze my fifth-great-grandparents, who would be cohorts Napoleon's generation. Of these I have the complete birth records for 58 individuals. They were born between the years of 1749 and 1793 and would have roughly been the same generation as Napoleon's. They died between years 1785 and 1865 with the youngest dying at 22 in childbirth and the eldest living until the age of 87, the average age of death being 68. Only 6, or around 10% made it to their 80s, of the octogenarians, only one was male, living until the age of 81. Only four seem to have been people whom would have been part of the upper classes, and with the rest being part of the peasant class . However, one thing to note is that there was a huge upsurge in deaths between 1808 to 1812,dates coinciding with the first and second French invasions of that region of Portugal (Santarem and Leiria districts) along with the aftermath, 14 out of 58 died in that period, or nearly one-fourth, so this seems to have skewed the life expectancy down. This includes one whom was shot by French troops in October of 1810.