I was 8. I know because something happened that year that caused my mental state to deteriorate from a chronic, low-grade depression to real major depressive disorder.
I didn't quite understand what death was or how it worked, but I knew that people who died were said to be in "a better place," and I was all for that. So I tried poisoning myself by drinking rubbing alcohol. I expected to drop dead in an instant, like cartoon characters when they drank poison. Instead, I puked my guts out after about one sip. It turns out that it really burns when isopropyl alcohol comes out your nose. Also, I didn't even die. Don't try this at home, kids.
It's interesting to see how very young most of us were when we first became suicidal. 85% of us were under 18. 37% were under 14. From the sound of it, some were under 8. I'd like to point out this data to the next person who hands me the "permanent solution to a temporary problem" cliche, but it probably wouldn't do much good. These are not people with a long and close acquaintance with reality.