Senile decay; infirmity due to old age — a general weakening of the body's functions associated with aging. Also used as the verb 'to die/decline of old age' (老衰する).
祖父は老衰で静かに息を引き取った。
My grandfather quietly passed away from old age (senile decay).
高齢の犬は老衰で歩けなくなることがある。
An elderly dog may become unable to walk due to old age.
長年の病気に加えて老衰が進行したため、入院が必要になった。
In addition to a long-term illness, senile decline progressed and hospitalization became necessary.