imho all you'd have to do to keep Peter 'young', is keep him and MJ on the low-to-middle rungs of their career ladders, and have random, occasional references to how they're a young, struggling couple. Just sort of... suspend time around them, and avoid stories that will have to be retconned in order to turn things back. Simply don't do those stories in the first place. There's enough commonality of experience from 20 to 30 that you could do an amazing variety of Peter and MJ stories (yes, I'm assuming that in the ideal Spider-verse they're together *g*) without aging them. They could go to school, go back to school, start their First Big Job, and lose their Big Job, any number of times without their seeming to age.
Peter being a high school teacher does not age the character. He could be teaching, what? Four or five years out of high school (depending on his program)?
As an aside, I'm in my twenties. I quite my Big Job to go back to school. Consequently I've got friends who are in school (undergrad and grad), friends who are married with children, and friends who are living through their jobs. They're all 'young'.