The most common is noise exposure over many years. Noise exposure can be as slight as working around computer printers or fans daily for many years; or specific hazards like dentist drill tools (dentists can get hearing loss from their drills!); or as great as working in a noisy factory or production area for many years. Recreational noise exposure includes shooting guns, working on car motors, woodworking, loud vacuums and hair dryers, concerts and loud music through headphones, and many other noisy hobbies and interests.
Another factor in hearing loss is your family predisposition. There may be a history of hearing loss in a family, and whether you protect your hearing or not, you may just get it anyway. I have a family history of hearing loss in my family, and I am noticing a high-pitched loss that is gradual.
One other factor in aging and hearing loss may be illness and treatments. Some illnesses (diabetes, kidney failure, Meniere's disease, etc) as well as some treatments (high doses of antibiotics, chemotherapy, and some other drugs) can cause hearing loss in some people as well.
Having said all this, I have seen patients well in to their 90s and even 100s who have very little hearing loss.
One important factor in diagnosing hearing loss is to have a baseline hearing evaluation. If you have not had one, ask your doctor to refer you to an audiologist for that test. If there is hearing loss now, you may want to have yearly hearing checks to monitor if there is a gradual decrease over time. Further, if you have significant hearing loss, you may be a candidate for hearing aids or other assistive devices.