I would use the term "childhood friend"

“Childhood friend” is good. You could say lifelong friend, but that would be more likely something someone would say when they are further along in life. You could be more precise and say “friend since kindergarten, friend since middle school ...” Note that a “childhood friend” could be someone who isn’t still your friend. Joe’s been my friend since my family moved to Dayton when I was four. Of all my childhood friends, he’s the only one I’ve kept in contact with, this despite our lives going in opposite directions.

Chum