Could this be because US gov healthcare spending takes place mostly via Medicare, which is specifically for people 65+?

Per capita stats are computed based on population, not per treatment or per person eligible for treatment. The US spends more per head on giving only a fraction of them subsidised healthcare than other nations manage on universal health, [more years of] education, [more generous] welfare payments etc. Some of that is down to medical professional salary discrepancies, but some of it is down to a terrible system...

The spending per capita, unless I’m mistaken, is just “per person in the country” not “per person insured by the government”.

That means that even though government insurance in the US only covers a relatively small percent of the population, we pay more than other countries that cover the whole population.