By WILL WEISSERT
WASHINGTON (AP) — President Joe Biden is celebrating his 81st birthday by joking repeatedly about his advanced age. At the same time, the White House is strongly defending his stamina and playing down polling suggesting that the issue could cost him votes during next year’s election. Biden is the oldest president in U.S. history. For months, he has been using humor to try to defuse the issue. But an August poll from The Associated Press-NORC Center for Public Affairs Research found that 77% of U.S. adults, including 69% of Democrats, viewed Biden as too old to be effective for four more years.