Are hemorrhoids normal at age 25 and is there any way to prevent them?

Could be. Hemorrhoids can be normal in adults, especially if you strain a lot with bowel movements, sit a lot or with pregnancy. Maintaining good bowel habits that encourage soft stools will help. You want to maintain good hydration and use stool softeners as needed. Increasing fiber in your diet can help as well.
No. No hemorrhoid are not normal but they can happen. They are congested blood vessels in the anal canal and are caused by straining to have a bowel movement associated with constipation the best way to prevent them or relieve the symptoms is to eat a high fiber diet with plenty of fluids.
Sure. Hemorrhoids can occur actually at any age; older people seem to be more troubled than younger adults. There can be multiple causes for this. Constipation, prolonged or undo straining on the toilet, obesity, increased abdominal pressure such as pregnancy, chronic diarrhea, and anal intercourse. So to prevent this bulging & painful swelling of the veins of the anus avoid these causes listed above.
No age limit. Most of the time hemorrhoids are due to bad bowel habits constipation, lack of fibre etc , best way to prevent the problem is to eat plenty of vegetables fruits , fluid, prevent constipation and local care , you will be fine make sure they are only hemorrhoids by seeing your doctor good luck.