senescence
noun [ U ] biology specialized uk/sɪˈnes.əns/ us/sɪˈnes.əns/
衰老，变老
the fact of becoming older, and therefore being in less good condition and less able to function well
Senescence refers to gradual deterioration with age and in animals is typically very obvious under zoo conditions. 衰老是指随着年龄的增长功能逐渐退化，在动物园条件下，动物的衰老通常非常明显。