Age restriction during registration in Magento CE 1.5

Hi, I have a Magento CE 1.5.1 store and I would like to restrict users from registering in our website whose age is below 13. During registration, they will enter date of birth and if they enter D.O.B. lower than 13, they won't be able to register and will receive a message that they must be 13 or above to register in the website. That's it. I hope it's a very small project. We require this to comply with COPPA (Children's Online Privacy Protection Act) rules. I need it urgently. Thank you. Regards.