(n.) The state or quality of being momentary; shortness of duration.

advanced age advanced years age of retirement an incurable disease anility changeableness childishness corruptibility death debility decline decline of life declining years decrepitude dotage dotardism eld elderliness ephemerality ephemeralness evanescence feebleness finitude fleetingness fugacity green old age hale old age hoary age impermanence impermanency incapacity infirm old age infirmity infirmity of age insipidity instability invalidism longevity momentariness mortality mutability old age oldness pensionable age perishability ricketiness ripe old age second childhood senectitude senile debility senile dementia senile psychosis senile weakness senilism senility senior citizenship shakiness superannuation the downward slope the golden years transience transiency transientness transitoriness unfirmness unsoundness unsteadiness unsturdiness unsubstantiality vale of years vapidity volatility wateriness white hairs wishy-washiness