The problem\issue\joke (if you want to call it that) is this: age ratings are usually there for some sort of reason, even though occasionally they seem rather lame. Now let's say some parents goes and buys game a game for their children after they are DOWANTDOWANTDOWANTDOWANT about it long enough, let's say GTA3\4 so for their 10 year old. Then let's presume the kid decides that woah it's awesome and then goes ape**** and does something nasty and he says "I saw it in a game", then the parents will raise a hell about it despite the fact they were the one ignoring the ratings in the first place.
When it comes to subjective point of view, some people might be fine and not have issues by game X even if they are notably under the rating, while some people who are adults still finds some stuff gross or inappropriate. Then comes the scenario where the parent is concerned that the junior might be seeing him\her playing something that isn't really appropriate.
We could also add the one odd variable that typically if E.G. someone sees something really scary or gross - whether it is a game or something on the internet - they rarely mention about it to their parents. It takes more of an exception for someone to talk about it rather than just keep quiet. It's not just about finding out is someone mature enough or not, it's combination of people being stupid and that not many can really expect what they're going to find behind the rating. I could use Silent Hill 2 as an example: the game was rated 16+ by PEGI IIRC yet if you can really understand what is going on you want to ask why there's no 18+ rating (as most of the stuff that's ****ed up is very obscure).
If you can play 18+ rated game without any issues while being notably under it, fine. There's plenty of people who can't and plenty of silly parents who don't even bother checking the ratings and then howl like it's the end of the world once they figure out the game had inappropriate material to their junior.
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