If you're truly confused about statutory rape, I think there are more questions to be asked.
There are way more details to this law other than a minor having sex with an adult, and unfortunately, a lot of people don't know them.
I think this website explains it best.
link
You need to take into consideration the age of the victim, the minimum age of victim, age differential, and minimum age to convict.
If you, or anyone, is personally worried about commuting this crime, I would pay close attention.
"Age of consent. This is the age at which an individual can legally consent to sexual intercourse under any circumstances;
Minimum age of victim. This is the age below which an individual cannot consent to sexual intercourse under any circumstances;
Age differential. If the victim is above the minimum age and below the age of consent, the age differential is the maximum difference in age between the victim and the defendant where an individual can legally consent to sexual intercourse; and
Minimum age of defendant in order to prosecute. This is the age below which an individual cannot be prosecuted for engaging in sexual activities with minors. The table notes those states in which this law only applies when the victim is above a certain age."
For example:
"In New Jersey, the age of consent is 16, but individuals who are at least 13 years of age can legally engage in sexual activities if the defendant is less than 4 years older than the victim." Because the age differential is 4 and the minimum age if victim is 13. So a 13 year old can legally have consensual sex with a 17 year old. And a 16 year old can legally have sex with a 20 year old.
"In Washington, sexual intercourse with someone who is at least 14 years of age and less than 16 years of age is illegal if the defendant is 4 or more years older than the victim. The age differential decreases in cases where the victim is less than 14 years of age (3 years), further decreasing if the victim is less than 12 years of age (2 years)."
"In Nevada, the age of consent is 16; however, sexual intercourse with someone who is under 16 years of age is illegal only if the defendant is at least 18 years of age (the age at which the defendant can be prosecuted)."
It's a very confusing law with many different variables, so if you truly want to understand, I would consider reading through that link I posted.
As for the maturity question, I don't think it has anything to do with being mature enough to give consent, but ones maturity in relation to their partner. Otherwise, the age differential wouldn't be in place.