import java.util.Calendar;
import java.io.*;
import javax.swing.JOptionPane;

public class Age{
    public static void main(String args[])
    {
        String year = JOptionPane.showInputDialog
        ("Please Enter Your Year of Birth:");
        String month = JOptionPane.showInputDialog
        ("Please Enter The Month You Where Born:");
        String day = JOptionPane.showInputDialog
        ("Please Enter The Day You Where Born:");

        int y = Integer.parseInt(year);
        int m = Integer.parseInt(month);
        int d = Integer.parseInt(day);

        Calendar rightNow = Calendar.getInstance();
    	int yearnow = rightNow.get(rightNow.YEAR);
    	int monthnow = rightNow.get(rightNow.MONTH);
    	int daynow = rightNow.get(rightNow.DAY_OF_MONTH);
    	monthnow++;
    	yearnow -= y;
    		if (m>monthnow || m == monthnow && d>daynow)
    		{
    			yearnow--;
    		}
        JOptionPane.showMessageDialog
        (null, yearnow);
    }
}

Copy of code hosted at http://pastie.org/684706
Code saved as Age.java