I was something like 8 or 9, maybe younger. The doctors (and my family) told me I couldn't possibly be depressed at that age and dismissed my requests for help as hypochondria or manipulation looking for attention, which only made matters worse. My symptoms of AS were also dismissed in the same way. I remember my mother always yelling at me to stop dragging my feet and being "lazy" when I just genuinely couldn't force myself to move anymore and would often simply break down crying.
I don't remember a time when I was "my old self" or feeling generally happy/satisfied with life. I have periods of feeling very happy, even euphoric, and (far more often) periods of horrible depression. When I feel the depression coming I do my best to fight my way out of it, getting exercise, forcing myself to *do* things instead of sitting around, but it's not easy and sometimes even when I'm feeling happiness from something, there is still that underlying "life is pointless and nothing matters" feeling. There is also a family history of depression, which probably contributes.