I own a bunch of the essential Spider-Man TPB books, so I've read all of 60's Spider-Man comics & most of the 70's Spider-Man comics. I've also read the first several "Ultimate Spider-Man" TPB books.
So then you should know all of this breh and it shouldn't be a surprise to you or a shock. So you know he tried or was recruited to other teams when he was in high school and continued when he was in college and nothing came of it until he was an adult. But also, they're doing civil war. for you to do that and not have him be a part of it is asinine. And as far as their other rationale, it makes sense. Right now, their movies are told from the perspective of really old men. Cap is ancient, Tony's got gray hair, Hawkeye has a family, Bruce is an old ass man who turns into a hulk and thor is a god. They're all adults or easing into another phase of adulthood. So having a kid be a part of that world offers something different. And for the way pete is manipulated during the arc, it makes sense for him to be a kid. If they made him an adult, then he'd have to have the adult motivations of a wife to protect and an aunt to protect and feel some loyalty to the guy who gave him a job and a place to stay when he was ass out and broke. By making him a gifted kid who still doesn't know any better but has a guy he idolizes take him under his wing, it feels a bit more powerful and also helps color Tony in a not so awesome light, depending on your perspective.