I've seen a couple of fics where Steve is really young when he goes off to war (somewhere between 16 and 18 years old). I want fic were after waking up and joining the Avengers Steve meets Spider-Man. By this point no one really knows just how young Cap is (18 years old at least, early 20s at most, can't be older than 23,) and they don't understand and Cap isn't likely to tell them. The last time he told someone was the Howling Commandos and there were all sorts of issues, mainly that they got super protective of him when he can take care of himself fine. When he first meets Peter he's out of costume, and even though he acts old fashioned and out of date Peter recognizes that they're around the same age. They bond out of costume as Peter teaches Steve how to act like a modern teenager/college kid. Eventually they find out about each others' alter egos and Steve promises to not reveal his identity if Peter doesn't reveal his age. And maybe when Peter is done with college he'll try to get him onto the Avengers. Things go great for a while until either one of Steve's teammates follows him and notices he's hanging out with a kid or the events of TWS happen and Steve calls Peter for help taking Bucky and Hydra down. After that I want some overprotective!team love for both Peter and Steve, and Bucky regaining his memory and being like "Great, there's two of them now."

tl;dr, Steve went to war when he was young and now that he's in the 21st century he's only in his early 20s. Out of costume he meets Peter Parker and the two bond while Peter teaches Steve to act like a modern kid. They find out each others' secret identities and that only makes their friendship stronger. Then someone finds out just how young Steve is and stuff happens, possibly TWS.

Basically Peter and Steve being besties. And the rest of the team seeing them as kids and being overprotective adults.