Age ratings are really complex, because it changes from country to country.
While partially nudity may give a "Teen" rating in some countries, in others it will give "Mature", etc.
I'm personally using the age ratings and content descriptions of my country, since I decided to focus only in the local market for the ease of administration purposes. Truth be said, how a single person can deal with global audience alone? Development, translation, localization, tech support, age ratings. Now multiply the amount of work by each language you decide to add support to (of course, if I had more persons working with me it could be done).
I think that if using something that already exists (like IARC) is a hassle, letting developers cope with age ratings at their own could be a better solution.
What could be a good solution would be to add a separate "age tags" section instead of a complete system with description and such. "Everyone", "Teen", "Mature", "Adult-Only" and "Non-Rated" for example. It still could lead to some headaches depending of the customer country though.