I live in Australia. The ratings here are pretty simple for games. Not even getting into that R18+ thing here, btw.
G - General
PG - Parental Guidance Recommended
M - Mature. Recommended for persons 15+
MA15+ - Restricted Mature. Not legal for sale or consumption for people under the age of 15.
R18+ - Restricted. Not legal for sale or consumption for people under the age of 18.
^ That system, as described, works well. I think incarceration is a little over the top, but a fine? Definitely warranted. Generally speaking, as far as I'm aware, it's the retailer who cops the fine, not the employee, but don't expect to see them at that retailer for too long.
That way, no criminal record for people who sell, but they do get punished. The system works pretty well.
...except for the problems like that our classifications board is inconsistent, pro-censorship, etc, but those aren't intrinsic problems with the idea of a restricted rating.
And, finally, this is possibly a problem - Mum can buy it and give it to kid. That's legally fine. Get a parent or legal guardian to okay it, and I'm not legally restricted from selling little Timmy the game. If Mum is there in person. Theoretically, there are laws about demonstration or consumption of restricted media by minors, but they're largely unenforcable. If we had a better ratings board, I'd be very happy with people not being allowed to view media above their age, but we don't, so it's a compromise.