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The calculus A/B here appears to cover stuff equivalent to an O level or gifted 15 year old/NT 16 year old in the UK.

I don't know anything about this, so I cannot say if this is correct. If it is correct, why is this so? Better math education in earlier grades? Is the UK typically considered a country with great math education? (Not asked snarkily, but I don't remember it being listed as such typically.) If calculus is typical at 16 in lots of countries in the rest of the world, then it's interesting that we are worried about calculus at 17/18 (heck, these days, 19) in the US.