By Kim Bhasin, Bloomberg News Each holiday season, thousands of teenagers tear gift wrap off shiny, new guitars. They giddily pluck at the detuned strings, thinking how cool they’ll be once they’re rock stars — even if almost all will give up before they ever get to jam out to “Sweet Child o’ Mine.” It may not be a big deal to them when they relegate the guitar to the back of the closet in favor of the Playstation controller.