Calculate Age Based On Dob Field?

I am trying to calculate age based on the date of birth input into a text field.
the date field should allow users to enter a one or two digit month, one or two digit date and a two or four digit year. Users should be able to enter any of the following date formats: 11-2-10, 1-25-2010, or 01/25/2010. On some days it works and others it wont, like 11-1-91 returns that the user is under 18.

function checkAge(){
var stDate = document.getElementById("birth").value;
var dayPartToDate = parseInt(stDate.substring(0,2),10);
var monPartToDate = parseInt(stDate.substring(3,5),10);

[Code]....