How do you still get in trouble if you smash a girl that claimed to be 21 & looks 21, but is like 15? (Have always been curious about this)?

Non American here... North Americans looks Way older than their age ( no offence) I've seen Girls who look at least 18 but they're something like 12 and 13 bro!! Fully developed and everything and mature persona/ethics ( hate to make it sound ped). I just find the law BIZZARE That if a female lies about her age and the age she gave you matches her looks and everything and both of you agree to smash... How is the guy Jailed and stuff when it isn't HIS fault? Same goes for stolen goods that you bought unknowingly... how's it the buyers fault? Is the justice system THAT messed up?

How do you still get in trouble if you smash a girl that claimed to be 21 & looks 21, but is like 15? (Have always been curious about this)?
How do you still get in trouble if you smash a girl that claimed to be 21 & looks 21, but is like 15? (Have always been curious about this)?
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