I was thinking about same thing recently too. I think it is easier to make friends when you a kid because of nature of kids. kids unlike adults dont have hidden agenda (well majority I guess), children pretty much honest and accept things the way they are. And as adult you kind of get to question people, especially based on not good experience so you either meet someone and click immediately or invest time to knowing person.
Absolutely agree about meeting right people. Those you usually don't even question when you meet them :)
Kids are more naive and less picky, this is why some childhood friendships don't last. As we grow we know ourselves better, and know better what type of relationship makes us happier and which don't.