MADISON, Wisconsin — President Joe Biden on Friday defended himself against mounting criticism over his age and mental acuity in a campaign appearance in Wisconsin, defiantly saying he won’t step down as the Democratic nominee and asking Americans to hold up his age against his legislative accomplishments and agenda. Biden asked supporters if he was “too old” to relieve student debt, appoint the first Black woman to the Supreme Court and sign climate laws. He also challenged the questions around whether he can continue to do so in a second term, when he would be 82 years old on Inauguration...