well im not 100% sure what the problem is but a buddy of mine was thinking it was the (int) in this line
private double calculateDogyears(int Humanyears, int Dogyears)
{
return 13 + (int)(16.0 / 3.0 * (Dogyears - 1)); *<-- this line :)
}
try removing that and see what it does for you. i didnt try running your code, but when i get time i will try it out. hope this helps