as a Rule of Thumb, anything you get from a form control is a string, so you have to take care if you use the + operator (it is used for addition and string concatenation).
you can make your life easier in the month case:
in the end, you just define a new Date object.
var day = document.form1.selDate.value;
var mon = document.form1.selMonth.value;
var year = document.form1.selYear.value;
var Birthday = new Date(year, mon, day);
after that you only need to calculate the year portion out of the Date difference.