Long-term studies show that both major depression (unipolar and bipolar) and mania are most common in early adulthood and less common in older age. The prevalence of mania tends to decrease with age even more than depression. Mood symptoms in general decline with age, and the balance does shift more to depression. It’s not that depression gets more common, it’s that mania declines even more.
It’s important to remember that those trends are averages. Every individual is different. Some people certainly experience more problems with depression, or more problems with mania, as they get older. But the average story is an optimistic one.
In some cases, specific medical problems or reactions to medications can cause mood symptoms in older people. When doctors see depression or mania for the first time in an older person—or see depression or mania getting suddenly more severe in an older person—they should look carefully for medical problems or reactions to medications that might be part of the problem.