There have been some very good posts on this site on this topic. Until I got Clifford I always had dogs and cats spayed and neutered quite young. Clifford is my first giant dog. After much research I decided to wait until he was 18 mos to 2 yrs old. There were too many possible growth problems and cancer risks and no real reason for me to get it done early. Annie and other dogs Clifford regularly associate with have all been spayed or neutered so no problem there. Well Clifford is getting neutered next week, he will be 17 mos old, almost made it to my planned age. He has done some marking and humping, and I have the time to do it now. Actually it's kind of funny watching him try to hump Annie. He gets her between his front paws and starts humping, well his back end is 2 or more feet away from her.
I don't know if it will help or not with the marking and humping, but it's time to do it anyway.