I am sorry to have to tell you that you are making one HUGE mistake in saying that when life expectancy in Malawi is 60, it means there are close to no people there 60 or older. The main difference is summed up in two words, my friend: Child mortality. Life expectancy at birth varies a lot, as your tabulation shows, but if you had looked at it once they have reached the age of say 20, it evens out. There are lots of old people in African countries, but the figures above are influenced by the terrible death rate of children.
Also, that a population is ageing, as in Japan and other countries, has nothing to do with people living longer, but has to do with the number of children born. Fewer children born = ageing population. People living longer does not influence the ageing of a population. People DO live longer, but it is two mechanisms that do not influence each other.