My answer is: No it's never too late, besides you may be overestimating your cat to be old. He's still young. I had the same problem with my 1 year old cat, and I fixed things myself.
Water spray isn't the solution here, in fact it may only make your cat sad for not being able to play with you and it may as well crack the relationship between you and your cat. Treats and small bites of food are good but at the right times. You should start training your cat to the word ''No''. However,do not hit your cat to stop it from harming you because that can make your cat either fear you or become more aggressive and bite more. And do not hiss at your cat except when it gets too rough for you to handle, and in your case I don't think your cat is aggressive I think it bites you but without aggression (correct me if I were wrong). So in that case you wouldn't need to hiss. Just say ''No''.
And here's how you do it:
- When your cat bites you hold his scruff with one hand, bring out your other hand's fore finger and say ''No'' (btw you can use any word instead if you like).
- Let your cat go and leave it alone and ignore it. If it bites again, do the same thing again, and if it does not get it, you leave the room.
- Do not yell at your cat. Make the ''No'' have a firm solid tone, stop whatever fun was going on. Because something wrong has just happened. By time your cat should be able to understand that when you say no you are communicating to the cat to stop because you don't like what's going on.
- When your cat is in a playful mood and you don't want it to mess with your hand, bring toys or whatever the cat likes to play with and have cat treats ready. Whenever the cat plays really good praise the cat with treats and use a praise word or a clicker to inform the cat that he's done well and he gets his reward for it.
- If you want your cat to really never bite you then never play with it using your hands. Use toys or treats instead.
Cats don't like to hurt their partners, it's just natural for living beings not to want to hurt beings who are close to them. However they may sometimes (especially young ones) not be able to control their energy and if it's not spent on something it may turn to aggression, so that's why we play with them. You should have play seasons with your cat daily.
Hope it goes well.