Allow me to explain my position a bit more.
If your parent/guardian is informed and educated about game ratings and still feels you can be trusted to enjoy it, great. They should get it for you.
If not, ratings should be followed.
"But my parent/guardian isn't informed and educated about games or ratings!!"
....Well, rather than whining about it, why don't you do something about that. Educate them. Show them, tell them, explain what game ratings are used for. Explain to them exactly what the game you want includes and defend why you should be trusted to play it.
You know, like an adult.
By demanding to be allowed to play a 18 rated game, you are asserting that you possess the maturity to handle the game's content. Rather than throwing a childish tantrum, prove that you have such a capacity.
Regarding the dangers of ignoring rating systems I leave you with
this, which does a good job explaining the danger American Gaming is facing in the political arena right now, and why it's important for gamers to portray themselves as mature, responsible individuals.