Can't remember ever not writing for pleasure in some way, so pretty young.
Overactive imagination needed an outlet, so was always encouraged to get it down on paper.
(I think my constant oral storytelling wore thin for long-suffering friends/ younger brother/ pets/ the postman/ random people in shops...).
I remember my brilliant Grandpa insisted he transcribe some of the more creative tales into mini books for posterity.
He and I also used to mail (scribbled, in my case) stories back and forth for years.
When slightly older: loved writing little sketches and performances based on book/t.v. characters for schoolfriends to act in.
Journaling. Penfriends (well, it was the '80s
)
Interesting that even when so young, I saw a very clear boundary between fiction attempts I felt comfortable sharing with others, and private writing just for myself.