Comment by close04

Anything that happens after the end of an individual’s reproductive years (perhaps also the period of raising the offsprings) stops exerting any evolutionary pressure. The fact that you have a “defect” at old age no longer blocks you from transmitting it so it can propagate through generations without evolution selecting against it. Random mutations “fixing” the gene can occur but still have to compete with the overwhelming number of non mutated “defective” genes.