Yes, generally vets will wait until a male kitten's testicles have descended before performing a neutering, because otherwise a much more invasive and expensive procedure would have to be used. The testicles of most males will descend by 8-12 weeks of age. In the rare cases that the testicles never descend, problems with the physical development of the hip/groin area are usually involved - and this would have been made apparent in an irregular alignment of the back legs.
The testicular sac can develop ahead of the testicles descending, which makes visual confirmation difficult. In such a case, you would have to manually palpate the testicular sac:
Hold the kitten belly-up in your lap (scruff lightly if needed) and gently examine the sac with your fingertips. Do you feel something like 2 little peas?
A vet of course can readily determine if you kitten is OK for neutering, and 12 weeks is a good age to get tested for FIV/FeLV and the 1st round of vaccinations, so a vet visit at this point would be a good idea.