You know, I know that you know that this is information widely disseminated, including from experts, and know you are not presenting it as your own opinion.
However as personal opinion I think it's silly and really wonder how validated, if at all, it actually is.
It certainly seems to assume great decrepitude with added years.
For example, I am 46, so I tried the formula with that:
X = 174
.85 X = 148
.55 X = 96
Now really, it is a reasonable and sensible lower limit for me to consider satisfactory and productive a mere 96?
Is a 46 year old necessarily THAT decrepit?
I am not.