Also, taxes and pensions are very small problems compared to uninhabitable coastal regions, mass migrations and food shortages that climate change predictions show us.
Age pyramid? That's not the biggest issue in China's demographics: it's the sex ratio. Talking vague FUD about China isn't helpful and doesn't contribute to this discussion.
Mass migrations and food shortages are happening now, but not as bad as they will be. This is due to both climate and US Monroe doctrine meddling (crime caused by the War on Drugs and installation of right-wing leaders with death squads).
I'm pretty sure that's because of Missing women of China[1], not necessarily the one child policy.
[1] See https://en.wikipedia.org/wiki/Sex-selective_abortion#China and the other linked articles.