There's really no precedence as football probably has never seen a guy who at 16 was bullying grown men around like it was nothing, the same strengths however lead to him developing serious weaknesses in his game. If you want a comparison the closest you'd get are the youth teams from around the Gulf of Guinea (mainly Nigeria) who fade badly after promising early careers; Lukaku is obviously not playing against children but I predict a somewhat similar development.
I'm not saying he's going to implode, he'll surely carve a solid to good EPL career out for himself. There are many types of strikers with different skillsets, in my experience however those that rely overly on their physique rarely make very good high end strikers and they often get found out at top clubs too (Benteke at Pool, Bony at City, Gomez at Bayern etc). If we're getting a new striker for 50m+ might as well do it properly.