Contrary to the myth that ageing is depressing, many studies find that happiness levels by age follow a U-shaped curve, with self-reported levels of happiness at their lowest at age 40, but then growing thereafter. The recent best-seller The Happiness Curve confirms this.
The Happiness Curve: Why Life Gets Better After Midlife - Jonathan Rauch (Bloomsbury Publishing plc, 2018)
Having said this, depression is an important problem among older people (as with other age groups). It is reasonable to assume that this has innumerable individual causes, and that one contributory factor is a society which does not value older people, rather than age itself.