Some people go to university when they're older. Some take time off. Some can't decide on a major and are in school for a decade. Plenty of explanations.
agree with your point, but "classmate" is often a reference to "a member of the class of [graduating year]", so it's ambiguous. even to my point, people can enter college almost anytime, not just right after high school.
It could mean same graduating class, or that they shared a class together. I probably should have written "overlapped at Harvard" as apparently they entered a year or two apart.