First of all, 10X is not a factor of age! "X" was meant as an arbitrary monetary unit, and the cost is the same 10X for both the kid and the 90 year old.
Second, "returning the investment" doesn't mean "reaching 100 years old". It just means "getting more years of life for the same cost".
Even if the boy lived only to 30 or 40, he still lived 20 or 30 years, whereas for the 90 year old living up to 100 or 110 would be extremely rare.
We spend more on dog food than we do on millions of patients.
So, no, it's not a question of "finite resources".
Or, rather, it would be, if we were anywhere near exhausting our resources.
When the NIH (or equivalent) turn that into a C/expected number of years, they're doing a scarce resource allocation problem. Unless you have infinite resources to give every patient every available treatment, you have to do allocation somehow.
Giving a 10-year old a treatment that you'd deny a 90-year old seems perfectly sane and rational to me.
I can't tell if you're agreeing with the concept and just disagreeing with the terminology, or if you believe there's a fundamentally better way?
I literally cannot figure out where your math went off the rails.
So, your maths is incorrect, try "5x the cost". But I take your point.