Depending on your age, your mouthguard may need replacing fairly regularly. If you are still growing, new teeth will come through and move into position. So the mouthguard may become too tight or loose, and will need to be remade to fit the new shape of your mouth.
Adults may not need to have their mouthguards replaced quite so often. But they are like any other form of sports equipment and will suffer from wear and tear. It is recommended that you take your mouthguard along to the dentist when you go for your check-up, so it can be checked.