That assumption inspires me.
It discourages procrastination, and encourages me to let go of plans that probably won't happen, since I'm almost out of time.
“To achieve great things, two things are needed: a plan and not quite enough time.” ― Leonard Bernstein
It's not even that complicated - if you've lived to age 54, you've already lived long enough to not die for the first 54 years and so your life expectancy is higher than the default "no info" value of 73 years old.
According to https://www.ssa.gov/oact/STATS/table4c6.html, you have an expected 25 years left.
In discrete time it's very simple; you just roll the dice every day, and if they come up snake eyes, Zeus smites you with a lightning bolt.
Not on average. Life expectancy such as "73" without further qualification means "life expectancy when born today". Since you already made it until 54, your life expectancy is higher than 73 (probably somewhere close to 80).
If you are not convinced outright, think of what your life expectancy is if you were 74.
So by the actuarial life tables, he has 24.5 years left, and has had about 30-35 years of self-directed life so far, which gets him up to 40-45% of his life remaining.