What causes tinnitus that develops over decades?

Usually hearing loss. Tinnitus is usually caused by dysfunction of the hair cells in the inner ear. Usually there is some hearing loss at the same time. The tinnitus is a symptom that the hair cells are not functioning properly. If the hearing loss is greater in one ear than the other, you need to have it checked.
Tinnitus over decade. Tinnitus is typically the first indicator of hearing loss. Tinnitus that worsens or becomes more evident over decades likely means worsening hearing over decades. Age-related hearing loss, which can be compounded by work and recreation associated loud noise exposure. Best to see an ENT and get a hearing test.