First time I'm hearing of something like that, but it does sound like a great idea. I think it would work more for the younger audience, though.
Depends on the blog. Not everything on the internet is geared towards young people. For instance, someone over 55 has a blog about their vacation trips to various museums around the world and they like classical music. A little snippet of a well known classical piece, or something from 75 years ago to open up the blog would lead people to believe that it was written by and geared towards seniors.
People like to go where they feel comfortable. Where they are around people like them, both in real life and on the internet. Age appropriate music could go a long way in telling a first time visitor whether or not that site is for them.