Scientific breakthroughs have allowed humans to live longer than ever, and it's not impossible to imagine a point at which most people live to be over 100.

There are lots of things to like about a world where everyone lives a century or longer: having the opportunity to meet great-great-grandparents, keeping people around to remind us of the silly mistakes that we keep repeating throughout human history, giving people a chance to try out new careers and relationships late in life.

But when people live longer, they also tend to get more age-related diseases, like cancer and Alzheimer's disease. As gerontologist Ken Dychtwald explains in this XPRIZE video on the future of growing old, the latter problem needs to be solved quickly — or we'll all suffer.

He says: "The rate for people with Alzheimer's and related dementia over the age of 85 is one in two. And so, as more and more of us live very long lives, unless we have a breakthrough to eliminate Alzheimer's disease, it will be the sinkhole of the 21st century.

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 That's just one of the quandaries we'll face, according to Dychtwald. Among his other questions: How can we create a better and more efficient medical system for the elderly? And how will they find purpose in those decades of extra life?

"How do [the elderly]...essentially give birth to the next version of themselves? Which is going to be necessary if we're going to live these very long lives, because one dream for life doesn't go the distance," says Dychtwald. "You'll need to continually reboot yourself and reimagine yourself along the way."

There's one thing that he doesn't mention about a scenario where everyone lives extraordinarily long lives: the added population pressure at a time when the world is already overpopulated. That could one day prove to be a bigger problem even than Alzheimer's. 

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