Natural selection has not eliminated many harmful conditions and nonadaptive characteristics that appear during old age. This is because:

The theory of aging evolution

Explanation:

The theory of aging evolution seeks to investigate the reasons why living beings begin a degeneration process at a certain age, among the various theories proposed is that of Peter Brian Medawar proposed in 1952, who claimed that natural selection avoided these mutations malignant during the reproductive stage to avoid the reduction of the population, however once this stage concluded the selection allowed them to manifest themselves.