My DD1 (Year 8 ) has been learning guitar for a few years, and according to her teacher is around grade 3 standard. However, she refuses point blank to even consider taking any kind of test that isn't compulsory. As far as she's concerned, she's always being tested on subjects at school and she accepts that as something she has to tolerate, but she sees no reason to put herself under unnecessary pressure.
I find it disappointing in some ways, because I'm the sort of person that likes to measure my own progress, but I'm not going to push her. She always does well in school tests, but she has no interest in comparing herself with anyone else (admirable in some ways). Guitar is for fun, so what's the point of grade exams?
DD2, on the other hand, is desperate to take her Grade 2 instrument test