This is almost a FAQ. In lib-common.php, find the function COM_getYearFormOptions and change this line:
for( $i = $start_year - 1; $i <= $cur_year + 5; $i++ )
Change $start_year - 1 to whatever you want the first year to be. If you need to go back to 1999 you would end up with:
for( $i = 1999; $i <= $cur_year + 5; $i++ )