I'm sure teens find sites like YouTube more convenient because it's a well-known website that everyone goes to, contains a search engine for finding exactly what you want, and they don't expect the RIAA to be banging down the door just for going to it (though that may change in the future).
Streaming music is also convenient in other ways such as streaming radio stations that let you give thumbs up and down to songs so that they will play more regularly or never again while continuously giving a fresh selection of music on a regular basis without having to do any kind of searching at all. (Wow that was a long sentence.)
With that said, I still prefer "tangible" music and videos that aren't going to vanish on me because the user who uploaded the YouTube video later decided to take it down. I go so far as to download YouTube videos that I like and save them to my hard drive. I find it to be more convenient.