This is probably one of the best examples of a properly executed thread. A new member asks a question about a tag name the meta rating. The member finds this tag on a page with a pagerank of 8, and asks the question if there is some type of correlation. Then two moderators quickly respond explaining that the search engines do not use this tag for any real purposes, especially ranking purposes. I think its best for me to quote Bill Slawski, from the thread named Meta Rating Tag.
Since the days when that might have been a way to indicate to some software that a site was safe for children or not, there has developed a whole standard way of creating ratings. The W3C pages describe it, though it's actually thrid parties that issue the ratings.
Some of them issue their own labels and decide which ones are appropriate for your site. Some of them allow you to make that determination. One of the best ways to get one of these labels is through one of the links on this page:
This is one of the ones linked there that I've seen labels used from a number of times:
Search engines do some filtering, but I don't believe that meta tags determine how that is done.
They can help you create a rating label specifically for you site
Bill adds more later to the thread, so check it out.