Taking Date From Form Field And Checking If Over 18 Years Old?

I am having some trouble with my code in that I am trying to take the value from the date of birth field and check to see if over 18 years old. I they are not over 18, a window.alert dialog box pops up to tell them. Below is the check if 18 validation.

Script section in the document head:

//check if over 18
function overAge(){
var age;

[code]....

Code for body section:

Date of Birth: <br/>
<input type = "text" name = "date" value = "(mm/dd/yyyy)"/>