Dating is not illegal as its only a status and not an action. But the 17 year old would basically be daring and begging people to arrest him and face charges if he actually decided to date a 12 year old. So no a 12 year old should not be dating a 17 year old.

Is it illegal to date a 12 year old when your 18?

Dating and holding hands is legal. Any sexual touching of any kind is illegal. The 12 year old cannot legally consent.