Pinworms are particularly common in children, with prevalence rates in this age group having been reported as high as 61% in India, 50% in England, 39% in Thailand, 37% in Sweden, and 29% in Denmark. [1]

Remember that

  prevalence is the proportion of a particular population found to be affected by a medical condition (typically a disease or a risk factor such as smoking or seatbelt use) at a specific time.

So it is not just that percentage has had it at any point in their life, it is that percentage that has it at any time.

And yes, kids. Pinworm is literally called 'children worm' here.

1: https://en.wikipedia.org/wiki/Pinworm_(parasite)#burkhart200...