I am a 20 year old child of a happily married couple, and my parents started fighting over a stupid issue last year. They are still fighting on that issue, how can I solve their fight?You can't do anything about it. It is not your fight. You have no responsibility in the matter.
When they come to you and complain about the other they are being unreasonable. They should not do that. You need to let them know that it is not acceptable that they use you as a sounding board. You are close to both of them and neither of them have the right to go to you with complaints about the other. Both of them need to realize that what they are doing is unfair to you and unacceptable from either of them.
They need to work out their problem with each other, not by using you as a mediator. You have no place in their argument and they are wrong to pull you into it. They are disrespecting each other by talking to you about their complaints toward each other. They need to show each other respect if they want to solve their problem.