If you believe it to be true, do let her parents know about what she's saying. Whether she's dating a 24 year old or lying, they'll deal with her, believe me.
Keep in mind that there's nothing a 24 year old would want with a 12 year old except for something extremely illegal.
Make sure she's not getting herself into any trouble; Find out who this 24-year old guy is and make sure he's not some sort of rapist...
what country does she live in? and yes it is legal though many would not agree with her.