High school in the US (~ages 14-18) doesn't have one track, and calculus isn't required to graduate. There were around 800 students in my year, and I'd guess only around 100-200 took the class, and it was usually in the final year. So of the other group, the ones that went straight into working instead of college never had it.

Edit: to clarify, mine was in the mid-2000s, don't know if there have been changes with things like Common Core, but from what I've seen/heard that seems focused on much earlier years so I don't think it would have affected this part of math education.

It would depend upon the details of what is meant by learning calculus. Learning enough to pass the average high school class on it isn't much. Above average at math, but nothing very impressive. But the same language could also describe learning it at something closer to the level of analytics, having a much deeper understanding than the average class produces. The deeper the understanding, the more impressive the achievement, yet it would still be described in layman terms as "learning calculus".

If true, this makes so much more sense and is in line with whatever else we know of Einstein’s genius.

If not, it sounds like resume padding by someone who wanted to make Einstein seem even more impressive than he is.

Calculus was introduced around ages 17/18 (senior year, i.e. the last year of high school, aka 12th grade, or the 13th year of formal primary and secondary education that most complete, including kindergarten) at the locally-regarded-as-very-good US high school this "elder" millennial attended, and only for those seeking the variant of our high school diploma that was supposed to get us college-ready.

But it may have changed since then.