If SHTF comes, our Life expectancy will be minimal either way, anything can cause death, a broken tooth, a small Infection, or even an appendix or something similar.
If there is absolutely no Medical care left after SHTF, and there is no one left who has medicine or can operate, life expectancy will be 30 Years or less.
Modern People today who only consume FB and Cell Phones will all be dead after a week, death because they can no longer Live without the Internet and virtual Friends, death because these People no longer have any idea how to pick up a shovel, death because you no longer have more than 1 Day's supply at Home.
If SHTF comes, the State will no longer be able to Help, the first two Weeks will be hell on Earth, after these 2 Weeks, for example in Europe, 85% of the People will be gone, with or without a Nuclear Strike.
If we are Lucky (or Unlucky) to survive these 2 Weeks and there is no improvement from SHTF, we will have to flee villages or Cities because of diseases of Thousands of deaths and we will have to start again in a very hard Time, a time where there is no longer any modern Technology.
Those who can still live without Modern Technology will survive, but as I said, our Life expectancy will also only be short.