This would be better for a 10–14 year old in middle school or early high school.

enriquto · on July 3, 2020

Hmmm. There's no way my 10 year old daughter would read beyond the first page of that thing. For 15-16 it's great.

jacobolus · on July 3, 2020

This is more about level of preparation / past experience than age per se. The OP describes a “bright, self-motivated child in their late teens who is into maths” and is a few years ahead of their peers. The mentioned book might seem a bit easy or elementary for this particular kid. The two Berkeley Math Circle books might be better. https://mathcircle.berkeley.edu/books

You are right that a book aimed at well prepared Russian 12-year-olds in an extracurricular math circle might be fine for 16-year-old average American students.