It's not a joke. Losing 30 minutes of your life when you're 25 costs you 30 minutes when you're healthy and full of energy. The average person experiences all kinds of problems after 75 (hips are bad, diabetes, effects of stroke, and so on). Conditional on having to choose when to give up part of your life, which is the tradeoff I was responding to, virtually everyone would choose after 75 rather than a younger age.
To the contrary - In many countries, average happiness is higher when you are older, despite physical challenges. The lowest happiness is typically in the 30s and 40s.
If you want to approach it from a happiness utilitarian perspective, a year at 60 or 70 is usually worth more than a year at the lifetime average happiness.
I can't tell if that's a joke or not.