The economy would be devastated with the demands of the elderly. We have a bunch of baby boomers nearing retirement age right now. Picture all the boomers living to 120 demanding services, and voting it in through sheer numbers.
Population control would be instituted. If our lifespan doubled, we would also have a large chunk of people here. Just picture 50 percent of all people born in 1865 still alive. It would be a mess. We would probably be limited by the government to 2 kids max per person. If ya wanted to become rich, you could sell your child credit to a wealthy person, in exchange for them having more.
I am all for people living healthy happy lives to 90... and then they can kick the bucket. If we extend life too much further, I don't think we can deal with the consequences very well.
One quick last point. If you like social security or not, there was a logic to it, when it was introduced. They had actuaries go over life expectancy tables and then they introduced the age of 65. They chose that number for a bit of a cushion. The median age of death was 63 at the time. Over half of all people wouldn't live to see it, so it was a cushion for those who did. Sort of a lottery in a sense. More likely than not, you wouldn't see dime 1, but in case you did live that long, here ya go, a little money so you don't starve. The program paid for itself easily, because a good 60% of people never got a penny back. Flash forward to now, where we are going to hit a life expectancy of 80 fairly soon... and we have a problem. If we extend age out to 100 even, and we don't totally realign the way our society is set up, we will have big issues.