栄枯盛衰[eikoseisui] ups and downs of life
減衰[gensui] attenuation
減衰器[gensuiki] attenuator
古び衰える[furubiotoroeru] to waste away
弱り衰える[yowariotoroeru] to languish

神経衰弱[shinkeisuijaku] neurasthenia
衰える[otoroeru] to become weak
衰え果てる[otoroehateru] to be utterly spent or crushed
衰運[suiun] declining fortunes
衰躯[suiku] emaciated body

衰頽[suitai] decline
頽れる[sutareru] to go out of use
頽廃[taihai] degeneration
頽勢[taisei] one's declining fortunes
廃頽[haitai] decay

頽唐[taitou] decadence
頽齢[tairei] declining years
胡頽子[gumi] oleaster (any plant of genus Elaeagnus)
人倫頽廃[jinrintaihai] decline (decay, corruption) of ethical standards
風紀頽廃[fuukitaihai] decay (decline, corruption) of public morals