Soon after I turned 40, I started experiencing back problems. I asked my brother for advice (he is a medical doctor) but didn’t quite like what I heard. “Sorry brother, our bodies are just not built for us to live much beyond 40…” he told me. If you take human evolution as a reference, he is right.
For the first 59,900 years of our existence (out of a total of 60,000), people used to live only into their 30s, at best. During the 1000 or so years that followed, there were some modest improvements raising life expectancy to 40 years. Things really changed only in the past 100 years on the backdrop of major medical and sanitation breakthroughs. 
Even until the mid-1950s, the average world citizen would have been lucky to reach the mid 40s (what we now call ‘mid-life’ in Europe). Rapid and successful poverty reduction, especially in East Asia, brought the world average to 70 years today. If advanced economies are any guide, many of us will make it to the respectable age of 80. So it’s basically our generation that, for the first time, has the luxury of contemplating a relatively long life.
Sub-Saharan Africa is not there yet though and only catching-up slowly. Between 1980 and 2000, life expectancy stagnated at around 50 years, before rising to 54 years . The continent still has a 16 year gap to close to reach the current world average.
Kenya even regressed before moving upward again. Kenyans’ life expectancy is slightly above the African average (56.5 years) but it has lost ground since the late 1980s (then 60 years and not far from the global average). It declined in the 90s to 51 years, mainly due to HIV/AIDS, before recovering over the last decade.
If the current trends continue, Kenya will return to its historical peak of 60 years, which it achieved in 1987, only in 2017. In other words, on this metric, Kenya has lost three decades (more than a generation). This is in stark contrast to East Asia, where China for instance has been exceeding the global average in life expectancy for a long time, even though it started below Kenya fifty years ago (see figure). 
Figure: Life expectancy in Kenya – Catching up after losing three decades
Source: World Bank computations based on WDI
Since 1900, humans have begun to steadily live longer. Advances in public health – especially vaccines, penicillin, but also basic sanitation and hand washing – have driven a phenomenal reduction in diseases . Once countries’ health systems improve and their citizens live longer, they can begin to reap a demographic dividend.
What are the main drivers of life expectancy and what can countries do?
The single most effective lever is to reduce child mortality . In most poor countries, mortality is highest among young children (below the age of 5), especially infants (below 1 year). Once children reach the fifth birthday, their chances to live a long life improve dramatically.
People often worry that declining mortality will translate into an unmanageable population explosion. That fear is misguided, because birth rates also fall as mortality declines. Why is this? People will have fewer children if they can be reasonably confident that they will survive into adulthood. Also, people move to cities, economic growth takes place  and women increasingly study and work: the first child is often later and family size shrinks.
The drop in child death rates followed by a dip in birth rates generates a demographic bulge : this large “extra” population at working age represents an opportunity. It can lead to growth and poverty reduction if the economy can productively employ all the new workers that accompany the bulge.
Many countries in Africa are starting to experience this demographic transition: population growth remains rapid but mainly because people live longer not because they have more children. Africa is now gaining 22 million people every year which is more than the 18 million citizens living in an average-sized African country . In Kenya there are 1 million “new” people each year, and that number is expected to remain stable (under the twin effect of falling fertility and longer life expectancy).
In Kenya and Africa there are now more working age adults than children and retirees combined . With better and longer schooling, an ‘education dividend’ is also emerging. All of this is underpinned by a ‘health dividend’, which helps Kenyans and many other people around the world to live a longer and healthier life. It then just needs more doctors to take care of people with back pain.
Gabriel Demombynes and the World Bank poverty team in Nairobi provided valuable input.
Follow Wolfgang Fengler on Twitter@wolfgangfengler .