My pillows are so old. But I don't know that how often I should change them exactly. I also feel different types of pain in my head and neck. So, please give me some suggestion.

It isn't so much that your pillows are losing their support, but the bacteria that accumulate in the filling and the covering. I replace mine every six months, but I don't use a firm pillow, that is bad for your posture and neck.

Thanks a lot for your answer.

I wash my pillows every six months or sooner. Pillows get full of mites. Mites live in our skin, our eyelashes, all over. You can never get rid of them, but you can reduce them.

As for pain, I have a lot of pillows. I made some out of shredded foam rubber. You need pillows that are shapeless and that don't flatten. You need to pay careful attention to how much your neck is bent while sleeping. I have very soft pillows under my shoulders (pain there) and then my head is only a very little bit higher on another very soft pillow. I have a pillow for my arm when I sleep on my side, so that it isn't making my shoulders bend toward each other. When you sit at a computer, make sure you put your arms as far behind you as you can every hour, and rotate your neck, and put your head back.

I agree, about every 6 months

I also agree with all of you.