It's not really a Ponzi scheme. A Ponzi scheme pays interest to attract investors so it needs significantly more customers with each iteration. Madoff's infamous one paid out 12% annually, so it needed to grow by 12% each year just to achieve stasis (or it would have if cash outs equaled profits anyway, some years it was probably more or less). Social Security isn't a Ponzi scheme because it would work perfectly well…
This is not true. While it's true that infant mortality rates in industrialized countries have decreased a lot since the 1930s, so have mortality rates at every age.