CRM 2011 Date Difference (in years)

Good Morning,

I am trying to calculate the age of a person from the date of birth and am not having any luck.  I have found some examples on like but nothing that is working.  Can some please tell me how to make this code work?

function CheckDOB(){
debugger;
var reviewDate=Xrm.Page.getAttribute("po_dateofbirth").getValue();
alert(reviewDate);
var currentDate = new Date();
alert (currentDate);
var cycletime = math.abs(currentDate - reviewDate);
alert (cycletime);
}

Thanks, Dale