Magento - add required date of birth to customer attribute

Using Magento 1.7.0.2 - Need this project done within a few days. I need a customer attribute of Date of Birth added to each customer who checks out at our magento site. Currently customers can checkout as a Guest or a customer and I need to obtain their date of birth to make sure they have a birth date greater than 18 years old. If they put in a date of birth that shows them under the age of 18 year old it will need to display a pop up restriction page telling them "Sorry, we do not sell any products to minors." They have to have an age (based on what they put in) greater than 18 to finish checking out. The default display age in the attribute box should reflect today's date 18 years ago. The age attribute will also need to display in the admin section under the Sales > Order > Order View > Information > Account Information So admin can see the date of birth when talking to customer.