Hey Tom check out how they do it in the Canada's rating system, it might give you some ideas. I think if you just changed some things, you should be able to get away with it. It's just that you had it so close to the video game ones.
http://www.empiretheatres.ca/movies/classification_main.asp
G
General - Considered to be suitable for all ages. No restrictions on viewing. The material does not contain elements that might be too intense or offensive for younger viewers.
PG
Parental Guidance - All ages may attend. However, parents should know that the theme or content of the film may be more mature than in general films. It may be more appropriate for those 12 years of age and over.
AA 14
Adult Accompaniment - Considered to be suitable for those 14 years and older. Those under 14 years must be escorted by a parent or adult. Subject matter or language could be controversial or offensive. The film may contain scenes of limited violence, the use of weapons without excessive bloodletting, restrained portrayal of accident or disaster. There may be scenes of casual or brief nudity, romantic involvement that is not sexually explicit where participants are at least partially clothed; some coarse or vulgar language.
R
Restricted- Considered to be suitable for those 18 years and older. Those under 18 years must be escorted by a parent or legal guardian. The film may contain excessive use of foul or offensive language and may depict extreme violence, horror, graphic sexual activity.
XXX
Explicit Material - Stuff that should get whistled on Newgrounds because anything heavily pornographic is against the rules.