I'm looking for a Harry Potter fanfic in which Harry goes to the past and is 5 or 7 years old. He is found and adopted by Merlin and is stuck in the past and is immortal until his time approaches. He becomes a healer and is adopted by quite a few people including Arthur Pendragon and Morgan le Fey.
You might be thinking of Basilisk-born by Ebenbild. Here's the link to the story The only thing is that he goes to the past as a 15-year old. Otherwise, it seems to match.