We got our first baby neutered at 8 months. He never really marked indoors, just outside. However, the humping started when he was REALLY young-six or seven weeks old (I know, I know--I didn't then-DO now!) and he continued for several weeks after neutering. Now the only time he does it is to show dominance over our new boy, who by the way, is 5 mo. and has NEVER humped or marked inside!
