Random question

What differs adult fantasy from children's/YA fantasy? Or is there a difference?

I just told a friend that I don't consider Lord of the Rings a series for children, but I think The Hobbit is. (ETA: I worded the rest of that paragraph poorly, so I've edited it.)

Also, what exactly is the difference between sci fi/fantasy/action and adventure? For instance, with shows like Buffy and Supernatural, I keep hearing different genres being assigned. Some say fantasy, some say sci fi (which to me usually implies spaceships or something else that is  science based), some say it's just action.

I'd just like to hear your thoughts.