> For me, the biggest tell was how frequently older people report feeling completely at peace and ready to die.

That's because it's inevitable and at that point they've been sick or infirm for years to decades.

No one has run the real experiment because they can't: put that person in the body of a healthy 20 year old and see if they still feel that way. Except we already kind of know the answer because we regard being suicidal in your 20s as mental illness.