Why can't it be both? Youth and inexperience may not necessarily mean attempting the impossible, like menloparkbum suggested, but certainly being youthful means having a better grasp on what's going on. So it's right to attribute part of his success to youth: he made a product that appeals to the youthful market.
Similarly, youth is a good reason to think that Foursquare could be a threat. The people making Foursquare have already created a successful product in this market. They spent a few years working for a powerful company that didn't understand their ideas. So they're the people who hit the market before Sam did, and they're returning with a more refined concept than they had before. The fact that they're youthful and determined is why I'm watching them and ignoring attempts like Google Latitude, which practically scream sterile.
There have been several startups doing the same sort of thing, all failing. I'm still not convinced people want this (Location aware social networking mobile thingy). It's a shame you can't get traffic figures so easily for mobile apps - without user/revenue numbers it's all speculation as to how successful or otherwise loopt is.
I don't use my cell for networking. I don't like that constant connectivity. But within that market, I would suggest that the Dodgeball team is a force to be reckoned with.
Perhaps I'm biased because Loopt is ugly as sin, and Foursquare is slightly better.
Similarly, youth is a good reason to think that Foursquare could be a threat. The people making Foursquare have already created a successful product in this market. They spent a few years working for a powerful company that didn't understand their ideas. So they're the people who hit the market before Sam did, and they're returning with a more refined concept than they had before. The fact that they're youthful and determined is why I'm watching them and ignoring attempts like Google Latitude, which practically scream sterile.