That's not true. Plenty of people get sick on this mountain, it's not the norm and plenty of elderly and children climb this mountain, but there's definitely a

That's not true. Plenty of people get sick on this mountain, it's not the norm and plenty of elderly and children climb this mountain, but there's definitely a not-insignificant number of people who could not handle the altitude without acclimatisation, among which many who're in perfectly fine health. Your personal experience does not refute any of that, just google Mt Fuji Altitude Sickness.