text: what are the laws on teen sex in wisconsin? at what age are you legally able to have sex. and what are the laws about teens having sex with younger teens. the laws in wisconsin, please. in wisconsin the law states that anyone who has sex with someone under 13 is guilty of first degree sexual assault and anyone who has sex with someone under 16 is guilty of second degree sexual assault. so, you must be 16 to legally have sex and cannot have sex with any younger teens.