Actually, neither of you can read. The statement was "If Joe wins, he'll be 78, eight years older than the current oldest president (DJT, 70)."
Joe will never be eight years older than the current oldest president, unless he loses and in, in four years, Trump is succeeded by someone born in 1950. That person will, by definition, be the current oldest president (he will be the only current president), and be eight years younger than Joe (assuming Joe is still alive).