[TanksAndSpartans]

Here's one I don't think has been mentioned - movies, video games, and books (The theory is that kids are impressionable and something like this could capture one’s imagination for a lifetime)

Off the top of my head I'll just list ones that could spark an interest in the historical aspect of the sport (theory being that most collectors start with current items i.e. "packs of this year's cards" and eventually get into older items)

Movies
Field of Dreams
The Natural

Games
Micro League Baseball (You could have Babe Ruth, Ty Cobb, Christy Mathewson, etc. on your team and they were good!)

Books
Casey at the Bat
Ted Williams Art of Hitting (not sure on exact title)

For Football, the only historical based movies I recall focus on college football - Rudy and one about Tennessee's Richmond Flowers (There is a Nagurski reference in a movie, but it wasn't a football movie). There was a video game, but it didn't come out until 2005. It's possible to stumble on books - but in my opinion, you don’t see as many - The Dr. Z chapter I read on Marion Motley as a kid was a huge influence on me, but I’m pretty sure I’m in the minority.

Probably lots I'm forgetting on both sides, but I think baseball wins on this. I can think of a kid on my street who said his favorite player was Babe Ruth - I'm not saying this was common, but I can't imagine it happening in football.