I don't have much memory of my childhood - my memory has always been terrible, coupled with a mixture of trauma, dissociation and living as a NEET so thus a lack of anything noteworthy during those times.
That being said, I have a specific memory of me writing a suicide note when I was a child. I do not know exactly what age I was, however I moved out of the house I was living in at that time when I was ten years old, so it must have been prior to that age. I recall my mental health worsening at ten, as well, including suicidal ideation. So, while I cannot pinpoint an exact age, I am aware that I have been suicidal and depressed since I was a child, with ten years old being the exact age where my mental state began noticeably going downhill.
And I mention depression, for even separated from suicidal ideation, I still recall suffering from it, even if I couldn't identify it at the time. Specifically due to child neglect, at least prior to ten years old.