WASHINGTON (Reuters) - U.S. President Joe Biden turns 81 on Monday, a milestone likely to draw attention to his status as the oldest person to ever occupy the Oval Office, with opinion polls showing Americans worried he is too old for the post he is seeking reelection to. Biden has addressed those who worry that he is too old for the rigors of the White House with humor and an attempt to convince voters that his age and experience over a half-century in public life is an asset in tackling America's problems. "I know I'm 198 years old," Biden joked...