Get Age JavaScript

I’m trying to express my age on my ABOUT page using javascript so I never have to update it. So wrote a JavaScript function.

function MyAge (BirthYear) {
ThisYear = new Date().getFullYear();
YearAge = ThisYear - BirthYear:
document.write("I am "+YearAge+" Years Old");

It should work. Here are all the dates below and the intended result.

BirthYear = 1982
ThisYear = 2005
YearAge = 2005 - 1982
I am 23 years old…

To me this seems logical. Where am I going wrong?