It's not going to just affect 70 year olds. It's going to be a gradual descent downward. So just making up some numbers perhaps it's 40% or whatever of 60 year olds, 30% of 50 year olds, and so on. And then aggregated out across the population it's going to be a relatively high number, especially as fertility issues drive the average age ever higher.

Eep, just looked up the exact figure and yeah the median age is now just about 40. So if something affects 10% of people over 40 then it'd affect 5% of all people. So you can bring my made up numbers above way down, and still end up with 5%.