A 13 year old is not supposed to be hanging out on the streets getting chatted up by men.
She's supposed to be going to school, playing sports or doing academic extracurriculars, or doing her homework or her chores or spending time with her family.
Unfortunately, the internet is the new “hanging out in the street”, it’s very easy to make yourself something you are not, and if you don’t also engage in a good bit of real-life interaction, it’s pretty easy to be fooled about someone’s age.
On the other hand, if you have sex only with the person you legally marry, you should be pretty much immune to such attacks. I presume (maybe a bad presumption) that if the state agrees with you about a child’s age, if it turns out later the child lied, you should get some dispensation.