Widespread concerns about President Biden’s age pose a deepening threat to his re-election bid, with a majority of voters who supported him in 2020 now saying he is too old to lead the country effectively, according to a new poll by The New York Times and Siena College.The survey pointed to a fundamental shift in how voters who backed Mr. Biden four years ago have come to see him. A striking 61 percent said they thought he was “just too old” to be an effective president.A sizable share was even more worried: Nineteen percent of those who voted for Mr. Biden in 2020, and 13 percent of…