"At first, it was just a summer job. That was a few summers ago." - From Lars and the Cool Kids. Sadie and Lars read as like 19-20 year olds to me, easily. 18 at least.

He saw them unfuse at the end of Alone Together, though! He probably just remembered them.

When his parents sheepishly corrected themselves about the name, that sold it for me.

Yeah, but 1. Steven's shock at Lars's body in the very beginning could be interpreted a couple ways and 2. This is Steven we're talking about. You think he would freak out at Lars about that?