Revisiting Spider-Man's early days

So you HAVE been collecting the Spider-Man collectible comics that have been appearing in the Sunday Gazette, haven’t you?

Last Sunday’s featured “The Chameleon Strikes” story from “The Amazing Spider-Man” No. 1, published back in 1963. It features a fun encounter between Spidey and the Fantastic Four; Spider-Man goes knocking on the FF’s door in hopes of getting a job.

I’ve read the story many times before (and no, I don’t have an original copy of “Spider-Man” No. 1). But it never occurred to me until now: How does the Fantastic Four know that Spider-Man is a teenager under that mask?