Mine is almost 4 and still has a marking problem. I bought belly bands to use when visiting family and he follows me everywhere when I'm home, but I still occasionally find "accidents". I'm sure they aren't accidental, just his stubbornness when it comes to doing what he knows is right, because he will look so guilty until the spot is cleaned up, then he's all happy like it never happened.
And yes, he was fixed when he was 6 months old.