People are usually 15-16 in s4 (when most sit nat 5’s) but you can sit Nat 5’s up until the last year of school (s6) and it’s not uncommon to do so (many retake maths) but these are usually being retaken. It’s definitely worth doing Nat 5’s before highers as so much of the content is taught in Nat 5, and higher just builds on these topics.
It’s still mildly inconvenient to move to a Scottish school in s4/s5 because some content is taught s1-3, so you will have to catch up on that, but it’s not difficult and it’s easy to find what you need to catch up on but it’s just a case of doing it