What is the youngest age a child can be diagnosed with depression?

Depression: I know an infant psychiatry specialist who has diagnosed depression in infants. I have no idea how one would diagnose a baby.

In nature: Most failure to thrive babies are actually depressed. This can be seen throughout the entire animal kingdom.

Uncertain: It seems that there is agreement that it can exist and even be severe but DX is controversial. Treatment may be difficult, but symptoms are taken seriously. Your doctor and/or therapist will need to be conservative with medications in a young, growing, delicate body. Play therapy would be a safe choice for the younger child.

1-2 yrs is possible: Sx of depression can be noted quite young, often in traumatized kids or those with separation from caregivers.