My Lucky had 2 undescended testicles. I waited for a year because there was some confusion about whether he may have been neutered during surgery for a hernia before I got him (long story). Ultimately his behavior told me he was ALL boy and I took him to the vet at about 13 months to have him check again. I wish I'd done it sooner so it could have been easier on him at a younger age. Poor thing had three incisions each about an inche long since they needed to search inside the body cavity.

Anyway, you will get a variety of answers so you need to go with yoru gut. I have not noticed any adverse impacts of waiting other than he does still mark occassionally, but it is MUCH much less of a problem than before he was neutered.