Earlier quoted context omitted.
> This is a reasonable problem to teach students about for like 1 week at age ~10. I'm afraid you have a completely unrealistic expectation of the median student (which is reflected in your other comments as well). I have taught hundreds of students and I would think a handful of them could answer this at age 10. This type of question first appears on the Khan Academy in Grade 7, i.e. targeted at 12-13 year-olds. The…
«Add 7.1% sales tax to the $19.95 ticket» (or whatever) is a 1-step (or generously 2-step) arithmetic problem, of the kind most students in many parts in the world learn to understand conceptually at age 6–7 with no problem. It additionally involves multi-digit multiplication, of the type people typically learn by about age 9–10 (?). If students have not practiced solving problems enough to handle combining these thi…
Singapore was the only one I found where percentage increase is introduced earlier as a national standard.
It's mentioned in the curriculum for Singapore Primary 6, i.e. 11–12 year olds, for students in the higher of two streams for mathematics.