Deadlines

(Image from kotaku.com.au)

The math is irrefutable. Old people have less remaining time than the young. It would not be surprising if the elderly were more afraid of death, but according to psychologists [bold added]

if you look at the research, older people have less anxiety and sadness and more overall satisfaction

Plausible explanations are:
1) the aging process prompts the letting-go of not only material attachments but also the disappointment of dreams that will never be realized.
2) in every long life there are at least a few moments that inspire gratitude. Remembering them, a grateful person is a happy person.
3) a legacy (e.g., children, educating the young, or building a business) that engenders satisfaction.

No one likes "dead"-lines, but they do concentrate the mind.