Age is relative. Age is a state of mind. Both of these statements are true to some extent at any given time. Both are informed by age-related factors within and without what- or whoever is being assessed as being old.

That is, if I were a 36-year-old with a chronic pain condition, I'd feel a lot older than an otherwise healthy 36-year-old, and that doesn't even touch on mental health and general disposition.

And either way, if I were a 72-year-old - because why not, let's double it again - both of those hypothetical people would be youngsters, and an 18-year-old would be practically a baby.