Originally Posted by
Lee
The whole Spider-Man premise just works better with a teenager. Not fitting in with his peers, having secrets, an overprotective parent, a turbulent love life, being broke, being misunderstood, playing the clown, screwing up, trying to do the right thing but making his own life more complicated.
With a teenager it's easier to write those kinds of stories and easier to sympathise with Peter.
If those kinds of stories are written with an older Peter Parker, it becomes less sympathetic (see all the posts of people complaining about Peter being "immature", "a loser" and "a man-child").
The other option is to have stories where Peter has a stable life, a good job, has settled down, isn't such a wise-ass, isn't so neurotic, isn't so self-pitying and self-deprecating, isn't screwing up so badly. Essentially completely abandoning the original premise of the series for a watered down, less dramatic version.