The population of the US is not necessarily the same as the population of Google's users with personalized ad profiles in the US, even though it sometimes feels that way :). E.g. you go from 349,000,000 to 267,000,000 just by removing the under 18s.
With 250 M users in the US, we’d reach 401 bn, vs 403 bn which were their worldwide revenues in 2025.
I think you mistook the example of removing a single group as a tally of what number to use instead. The final number should be significantly less than the 250,000,000 who aren't children.