Keep in mind that when you ask your mom when you can date, she'll most likely respond with "Why? Do you like some guy?? Do you want to start dating?????" She'll suspect something, that's how mom's are. xD Maybe you can ask her when SHE started dating, or how she met your dad, and she'll probably somehow get to the point where she'll say, "YOU, young lady, will start dating when..." Just ask relevant questions.
How old are you talking here? If you're under fifteen, I wouldn't eve mention it. If you fifteen or older ask her when she started dating and who she dated. Then start making the conversation flip towards you.