1. Living longer doesn't mean your standard of living will rise. Even if you work longer, you will not necessarily accumulate significantly (or perhaps even any) more wealth. And if you are in poverty currently (ie, effectively earning nothing), multiplying that by another 300 years still leaves you with zero.
2. If there is any change to the accumulation patterns of money, I would suspect that either the rich would get (individually) richer -- or perhaps more money would be printed, thus de-valuing the existing money.
3. Being at the median global living standard does not guarantee that you will produce fewer children. For example, there are many people in Western countries who for religious reasons continue to produce large families. More time for them would presumably result in even larger families.
4. In fact, Africa is particularly interesting, because its birth rate is NOT dropping as fast as predicted due to economic development: http://www.economist.com/news/leaders/21598648-birth-rates-a...