1) He hasn't been a kid since ASM#28.
2) Back when he was a teenager, almost nobody saw Spider-Man as a teenager.
3) He's not stupid.
Remember that Spider-Man was one of the first superheroes in the so-called "Marvel Age" that began after the rocket flight of the Fantastic Four.
Spider-Man was an active superhero before Iron Man, before Captain America was thawed out, before the formation of the Avengers.
Here's Spider-Man's first encounter with Iron Man where he treats him as basically a loser salesman (which is what Tony Stark is if you think about it).
The Avengers #3 - 1.jpg
The Avengers #3 - 2.jpg
There were times when writers remembered this part of the continuity, like when Captain America offered Spider-Man to join the Avengers trainee program:
"Trainee program?!? Hey, I'm Spider-Man, remember? I was sticking to walls when you guys were still looking for a clubhouse, I'm no green rookie!"
-- Spider-Man, The Avengers #236, written by Roger Stern.