The study is based on 100 cells from one 74-year-old man. The follow-up study will involve 150 individuals. I hope there's a very wide age range among them. They describe much, maybe all, of the variation as error, but one could imagine some of the variation actually serving a useful purpose and therefore existing even in young people.
Isn't the effect largely an age + lifestyle effect? Mutations increase as one ages, and things like smoking increase mutations (as the Nature write-up mentions).