Senility, Senile Decay
noun, verbal noun, の adjective
An elderly person in decline is someone who is old and in decline. They are experiencing senility or senile decay.
ろうすい
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
母親が老衰で亡くなったとき、妻が「厄介者がいなくなってせいせいした」と独り言を言っているのを私は聞き漏らしませんでした。
When my mother died from old age, I didn’t fail to hear my wife mutter, "that parasite is gone, good riddance," to herself.