Definition
Peak Child is Rosling's name for the moment, reached in the 2010s, when the number of children under 15 alive on Earth stopped growing. It is now hovering around 2 billion and is projected to flatten or slightly decline rather than continue rising.
From here on, future population growth is no longer about more births per year — it is about the children already alive aging into adulthood and old age, a process called "fill-up."
Why it matters
How it works
Peak child is a consequence of the global fertility transition: average births per woman has fallen from over 5 in the 1960s to about 2.3 today and is approaching the replacement rate of about 2.1. Once average fertility is at replacement, each generation roughly matches the previous one — the children compartment stops growing.
Population still grows afterwards because each existing cohort moves up the age pyramid and lives longer than its predecessor. Picture three large generations alive at once today; tomorrow there are four; eventually the bottom of the pyramid stays the same width while the top fills in. The total grows even though the inflow does not.
The arithmetic settles around 11 billion by 2100 in the UN's central projection, with growth almost entirely from adults aging into older brackets. That is a very different problem from a runaway birth rate, and demands very different policy.