Although young children may feel fear, they do not experience anxiety or depression. And while they may express pain when their bodies are hurt, they do not complain of back pain. The decrease in anxiety, depression, and the like as one grows older is due to the development of positive experiences and the ability to make good predictions.
I believe that imagining an unpredictable future and discrepancies between predictions and outcomes can cause depression, but such feelings arise precisely because new events happen and people challenge themselves with new things. Therefore, depression in adulthood is itself a higher cognitive function and a sign of growth. So I think apathy and depression in old age are fundamentally different.
However, if depression persists in early adulthood, it can lead to decreased motivation and can also be accompanied by apathy.
Coping with the stress that comes from experiencing and challenging oneself is important during adolescence and early adulthood, because eliminating too much stress stunts growth.