This was that thing I was actually working on in my first livestream a month ago. I ended up sticking it in my AP portfolio so I wanted to wait until I sent that out to post it here.
If you think about it in terms of BBC canon, Sherlock would have been about twelve when John went off to Uni. If I ever were to write a kid!lock fic (I really want to, though it’ll probably be more of a drabble series!), the canon would be that John sort of befriended this strange kid genius through extreme circumstances, and just ended up following him around and assisting him from that point on.
It would be pretty sad when John inevitably left, but we all know they meet up again a decade or so down the line.