> doesn't browse Reddit, or HackerNews, or TechCrunch, even though he's into tech and startups
> he doesn't know most of the popular memes on western social sites
> doesn't use Facebook, at all
Is this what "American" means?
Sorry for not being able to provide better examples, but I guess if there is a Turing test for "being American", I think I would have done OK at it.
Immigrants make choices constantly -- subconsciously or otherwise -- on how much to indulge in the local culture, and how much to stick to your familiar roots. Unfortunately, there can be strong feedback loops that develop quickly; one immigrant group is tightly knit because they find it harder to bridge the gap and the gap gets harder to bridge because Americans find the immigrant groups "too ethnically knit" and closed to bother. Not to mention that there's a natural tendency to be a little apprehensive towards someone new with their own set of customs.
My examples were very limited, they are only used to illustrate a point, but not to make a broad statement.
Maybe at 14, your friends aren't locked into FB/WeChat/etc, and the network effect isn't as strong to attract you to one cultural set or another. The other theory I have is that with how much larger colleges are, and in particular how many more international students there are at the collegiate level, there is less pressure to 'assimilate' and find 'American' friends. I've definitely noticed that international students hang out with other internationals a lot in college. It's really common to hear Portugese or Korean in my library, for example. Having some support base probably makes it easier to hang out only with those who are super similar to you.