11 or under is considered rape and child abuse, not just taking advantage of a minor.
To the OP: If you want to sleep nights with your boyfriend, and this is what you want in life, you will have to apply to a court to be an emancipated minor. Otherwise you have to do what your mother tells you because it's actually what is best for you right now.
You may know what you want in life right now, but so does a baby when he wants to eat and so does a 5 year old when he wants a bike. You're perception of life is going to drastically change over the next few years, especially if you become pregnant, which could very possibly happen whether or not you are using safer sex practices.
For you to be sleeping with someone who is 20 is something that is illegal for him to do, and he could go to jail.
Slow down with your life before it's too late and you have to grow up faster than you're meant to.