Within science overall, experts are very reluctant to risk over-promising & under-delivering ... Whenever they have the misfortune to be talking to the general public, they want to say 'We don't know.' In most walks of life, saying you don't know is the opposite of what you want to do; you want to pretend you do know stuff that you don't know. In science, it's the other way around; you pretend that you don't know stuff that you actually do know.

Machines are set up to tolerate a certain amount of damage without any really appreciable decline in function. So, this is why the health problems of late-life are problems of late-life and we do not see them at all until after middle-age ... Even a 20-year-old or a 25-year-old has some—quite a bit, actually—of damage in their body, but you wouldn't know it.