I see from comments on this post that some readers are missing the point, in a couple of ways.
First, my beef with the “life expectancy was only 68 years!” thing is that it’s meant to convey the impression that in the good old days the typical Social Security recipient only collected three years of benefits before keeling over; in fact, it was more like 14 years, and has now risen to 18.5. So yes, the length of time benefits are collected has risen, but not as dramatically as the usual suspects would have you believe.
Second, child mortality really is irrelevant: someone who dies at age 7 neither pays into the system nor collects benefits.
Third, yes, death in working years does have an effect: if a 57-year-old dies, he or she has spent 35 or so years paying in, but never collects benefits. That’s why a return to the Mad Men era, with lots of smoking by working-age Americans, would be good for Social Security.
The point, though, is that all of this is folded into the SSA projections, which show benefits rising from 4.8 percent of GDP now to an eventual plateau around 6 percent of GDP. That’s not peanuts, but it’s far from the apocalypse you keep hearing about.