Yes, it can go bad. Spoiled paint will have a sour milk smell. I've also noticed a very strong ammonia smell in old paint but I'm not sure if that means it's gone bad. Probably does.

Most paint will be OK for three years if stored properly, which is at temps between 50 and 110 or something like that. Freezing is bad and heat, like in an attic, will shorten the life very quickly.

But I've also read that five or six years is ok for some paints. I just mixed up a bunch of old cans of paint, some of which were six years old, and it seemed fine. I painted a large room with it and it went on the same as fresh paint except that I should have strained it first as there were a lot of small blobs of thicker paint in it. But other than that it seemed fine.

Thanks.

"... very strong ammonia smell..."
Ammonia is used to keep the latex from bonding in the can. It evaporates quickly after application and allows the lates molecules to join together into a solid film.
There will be a strong smell immediately after opening a can that has air space and has been sotred for a while.
Use some of the paint on a test piece to check if it will form a film and dry correctly.
Mistakes are a PITA to remove from the walls.

Please help! I think I have used off paint!

It didn't smell when I opened the tin, in fact the smell has only emerged now the paint has all but dried on the walls! It's like off milk!

I have used it in a walk in cupboard/wardbrobe that previously had damp and was left with a musky smell. I did an undercoat of a damp seal paint, which was fine and left to dry before using this kitchen/bathroom paint - I thought for added protection, not to smell worse than before!

Will a coat of a different (and not off!) paint hide the smell or do I need to do anything else?????

Any advise really welcome!