That is going to be a fairly large thing to unpack
I probably should start with that in the UK there is the BBFC rating system which is legally enforced (bar a small quirk the other year). As in the courts and police have recourse if a vendor sells my 17 old self an 18 rated game. Theoretically the ratings are the same as those for films/dvds, in practice they are more prudish and sensitive to violence, though the prudishness is nowhere near US levels. You can be refused a rating entirely (Rockstar's Manhunt being a famous example) which then actually makes it illegal to sell in the country -- AO in the US means a lot of vendors won't stock it and console makers won't allow it but does not make it illegal. Equally some things can attract different ratings -- much to my annoyance Perfect Dark somehow got an 18 rating in the UK, though I see it also won an M in the US. Demolition man got a 15 rating in the UK.
In my experience the BBFC rating is not as strictly observed for games (it is just games, plenty of times I have been in game shops and GTA has gone out to kids because a parent bought it for them) by anyone really.
There is also the pegi rating system (pan European something) which sometimes comes back with different results to the BBFC one, this is not legally enforced that I know of but some places might anyway (I got refused once when I was younger for a pegi rating, though I wonder if it was not the clerk trying to prevent me from playing a bad game).
The legally enforced thing does not seem to bother anybody in the UK, compared to the US where various states have tried to bring it in and various politicos have also tried it on at times and people get very up in arms about it. Though the making illegal refused rating does annoy people on those occasions when it happens, it is not as bad as the "video nasties" era though.
We will also have to look into the company policies -- North America Nintendo was famously ultra prudish and censor happy in the NES era and beyond. Nintendo of Europe at the time was far less concerned with such things, many things trickled down from the NA division though. Sega, who in various parts of Europe was as popular or nearly as popular as Nintendo, did not care nearly as much and if you look at some of the native stuff like the Amiga then it is almost a free for all.
I saw a fantastic documentary once called This Film Will Not Be Rated that dealt with the general odd approach to life the MPAA favoured. I suggest that if you are looking into this sort of thing.
