I'd rather ask: who didn't? The only reason to not come to school on your own by that age is if the school is far away and hard to get to by public transport, which usually isn't the case.
Where do you live? There are many, many places in the US where it is indeed usually the case. (Possibly depending on your definition of "far away")
(for the record: primary school encompasses what US calls elementary and middle schools here)
My very liberal definition of "far away" would probably be something like "more than 1.5km" (the distance I was going by foot to a swimming pool as a kid), more if we consider public transport and bikes too - though those of course won't necessarily be viable options everywhere and at every age.