Why are so many US politicians so old?

This week two key figures in US politics are generating headlines, in no small way, because of their age. Yesterday, House Speaker Nancy Pelosi announced her plans to step down as leader of the Democrats in the House. At 82, Pelosi said the hour has come for a new generation to lead. However, she will keep her seat and Congress with no plans to retire.
On Tuesday former President Donald Trump made it official, announcing he is again running to be President of the United States. Trump is 76. He would be 78 if the voters gave him another chance to move into the White House. Trump will be spending the next year or two on the campaign trail - no retirement for him, either.
What about the man in the White House now? Joe Biden has a birthday coming up. On Sunday, the president will turn 80. Some consider Biden too old for the job.