In 2011, cartoonist Randall Munroe published a graph showing the number of living humans that have walked on another world. It rose in 1969-1972 to a peak of twelve Apollo astronauts, then declined in the nineties with the deaths of James Irwin, Alan Shepard and Pete Conrad.
In the current decade, with the deaths of Neil Armstrong (2012), Edgar Mitchell (2016), Eugene Cernan (2017) and John Young (2018), the number has fallen to five living astronauts. Randall Munroe predicted in 2011 that the last man would die between 2023 and 2035 with 90% confidence.
Several countries (as well as the company SpaceX) have announced plans for manned landings on the Moon or on Mars in the 2020s or 2030s, which would increase the number for the first time since 1972.
Will the number fall to zero before the next increase?
Negative if any human lands on another planet or a moon and walks on the surface while there is still a living Apollo astronaut who walked on the moon.
Positive if/when the last Apollo astronaut dies, otherwise.