@pcj It's probably planned to be added at some point, but seeing as how age rating systems for books is based on what the publisher decides rather than some sort ratings agency or governing body it may be lower on their priority list, especially since there's no filters for age ratings at the moment.
Rather than a pure suggested age filter (which may be applying a specific country's standards), I'd like to see something underneath that indicates what in the book got it that rating assuming it's not some sort of "E for Everyone" rating. For example, Suggested 12+ for Mild Fantasy Violence. That way people can see it's rated whatever due to violence, gambling, sexual situations or whatever since different people place different emphasis on different particularities.