Double stat rape?

Different states have different precise laws on this, but generally most require a specific age difference between the two participants, which is gender neutral.

In other words, they tend to define "sexual activity" in one section, and then in another state that if a person has "sexual activity" with someone under the age of X, and the accused is more than Y years older than the 'victim', it's a crime, regardless of consent.

Sometimes it is a defense to be unaware of the age difference, but in some states, a man can get 10 years for having sex with a 15 year old that they met in a bar where you might think he could presume she was 21 because of the liquor laws.

There are 50 DIFFERENT state laws on the issue in the US. Because you refer to 'western laws', I will assume your question is not restricted to the US. I have never heard of a case in which BOTH minors are charged and would be surprised if it ever happens. The first answer is incorrect in at least one regard. Under EVERY US statutory rape law, ignorance is NOT a defense. Even if the minor shows the other person a false ID stating they are not a minor, it is STILL statutory rape. Also note: many states don't actually use the term 'statutory rape'.

no - generally if the kids are close enough in age, then stat rape laws don't apply. you have to read each state's law carefully for the specifics.

Statutory rape is when 1 person can legally consent to sex, and the other can't, so if they are both underage, it is not statutory at all.