What's the obsession with Pokemon?

Lately, my close friends have heard me make many comments about my brother, Jay playing Pokémon. It's quiet funny because for a while they thought I was joking. They thought "playing Pokémon "was secret code or a slang term they didn't quiet understand. Strange enough, it isn't. I have spend multiple nights and days watching/listening to my 18 year old brother lay on the floor of our family room with his best friends and play Pokémon. I watched him sell his Xbox which I thought he loved more then his IPhone to buy a new Nintendo DS (it's cool PS3 is better anyway).
I am totally okay with the idea of my brother playing Pokémon instead of first person shooter games like Call of Duty but what I don't understand is what is so great about Pokémon that it obsesses people over thirty and to play. Did you know there is even a Pokémon Video Game Championship? Evidently, players from all across the US and Europe gather together to 'battle.' 
Let's look at it psychologically. Pokémon is a fantasy game, it falls under the same category as Star Wars, Harry Potter and Lord of the Rings. The legendary Pokémon story lines also give it a mythological edge. There is also the idea of obtaining a collection. People love the idea of collecting things thats why some people are hoarders.
What I question the most is why do people not stop playing after a certain age? You figure with most things in life, Pokémon players would just evolve and start playing something more age appropriate. I believe the reason this doesn't happen is because there has never been an actual age limit applied to Pokémon. Like most things in life people aren't afraid to say "you're to old/young to do that."
I'm going to assume that as my brother and his friends gets busy in college they will stop playing but for now I hope he continues to play and enjoy life while he can because life after college sucks.