At 14, if the kid is very strong at math, they should try first year university classes. There are tons freely available -- just poke around university math dep

At 14, if the kid is very strong at math, they should try first year university classes. There are tons freely available -- just poke around university math dept web pages (e.g. Oxford, Cambridge, Berkeley, MIT). Some have full videos, some have full lecture notes, some have exams with answers, etc. For a 14 year old I'd recommend US universities as the first year is at a lower level there than UK universities (don't know much about non-anglophone universities). E.g. look for classes called "Precalculus" -- that will not be too difficult for them. And also introductions to mathematical logic (and set theory) to get their math thinking off on the right foot. The other advantage of this is that it will get them in the mindset that a university environment is where they're going to be in their 20s. Doing an undergrad degree and then probably a masters/PhD, which it sounds like is what this teenager should be starting to think (not yet, but when they're 16/17).