Okay, I did look up some statistics. Did not find life expectancy at
age 18, but did find age 21. Close enough for government work. The
source is the Social Security Administration.
http://www.ssa.gov/history/lifeexpect.html

In 1940 53.9 % of those males that had reached 21 also reached 65. Or
46.1 % paid into Social Security and did not collect any retirement
benefits.

In 1990 72.3 % of those males that reached 21 also reached 65. Or 27.7
% paid into Social Security and did not collect any retirement
benefits.

You may not think that 27.7 % is a large percentage, but I do.

And the change from 46% to 28% is kind of significant too.
This means that 18.4 % of the male population went from receiving no
retirement benefits to receiving an average of 15.3 years of benefits.
Which is why I say that you want more than just the life expectancy at
age 65.

Dan