Ideology aside, I consider Biden too old for the job at 78. In 4 years, Trump will be 78 and I will consider him too old for the job as well.
I love what he did for the country during his term, but the temperament of people does NOT improve with age and he has already been on the edge.
That entirely depends on the person. Although only four years younger than Biden, Trump seems 20 years younger in terms of mental acuity and vitality.