It's just a thing of personal preference and doesn't have anything to do with age. Some people like light-hearted movies and games. The cartoon look just has a special feeling to it, it's abstract, it lets your imagination go wild. Some people prefer realistic topics, which is fine. But you should never ever judge someone just for their personal taste.
For a really long time I only played games with a somewhat realistic look. Then my son came along. Because Battlefield or Gears of War isn't quite the right entertainment for a new-born, I got some cartoony games over time. And I didn't expect it, but I enjoyed them like there's no tomorrow. I also discovered various cartoon shows which I really like. Gravity Falls is just great and I also absolutely love Phineas & Ferb. That stuff is hilarious!
Today I play Minecraft together with my son and I absolutely love it. In fact, it's way more fun than sitting alone at my console fraggin' other people in Battlefield or something. I'm really glad I made that experience. It really showed me that games with a "kiddy" look can enjoy anyone in any age. You just have to let them.
It's also the reason I am really looking forward to the next Nintendo console. I mean I still play realistic games. But right at this moment, "childish" games entertain me a whole lot more. If that means that I "never grow up" as someone mentioned in this thread, then so be it. I don't care as long as I'm having fun. Gaming is about fun for me, not about lifestyle. I don't have to make a statement with it. I just want to have a good time.
Official member of VGC's Nintendo family, approved by the one and only RolStoppable. I feel honored.