I've listened to old radio show episodes - Jimmy has a much bigger presence there than in golden age comics. He started as a kid and grew up along the way, being close to Clark the entire time, as I recall. And, of course, he had his own title as Superman's Pal in the silver age (and beyond). But I'm curious as to when we started seeing him as a friend to Clark Kent, spending time with him (especially outside of work), etc. I'd think part of that is the shift to longer stories and more panels, allowing us to see some downtime (scenes not directly relevant to plot), but I'm just not sure.
I'm also curious as to how his age shifted around in the comics, and how that might have affected the dynamic.