When applying for a PhD is it worth mentioning awards received for achievement in final year high school exams?
High school awards are unlikely to help much but they probably won't hurt you either. I think it's fine to include them on a short "early-career" CV but move them to the very end to signal that you understand that these are not important measures of your current qualifications.

Who knows, if they are really awesome, they might might even help. I saw a graduate student CV once that listed being ranked first on a province-level college admissions exam that included many tens of thousands of students. Sure, it was a long time ago, but I was still impressed enough that I remember this now. I think I kept my high school awards on my CV until I started thinking about going on the faculty job market as a PhD student. At point, they just seemed completely irrelevant.