My grandmother suffered with bi-polar most of her life. Had an early diagnosis. Went in and out of hospitals starting from a young age all the way up into her late 50's.

She received ECT in her last hospital stay and was released on a certain regimen of meds one being lithium.

About 5years later my grandfather passed. She maintained. As she got older she did fine. In fact, her dosage on lithium got smaller and smaller.

This isn't to say that everyone will get better with age, but I would think that every case is different.