The term ''youkai'' can also be used to refer to subjects and topics related to youkai in general.
All youkai species are reputed for their longevity. Although they mature and reach adulthood at the same rate as humans, they can maintain their adult bodies for an extremely long period of time, with an average life expectancy of about 300 years. Individuals reaching 400 years or more are extremely rare; the only exception to this general rule are a [[kitsune]], who cannot die of old age.
Note: "Youkai" and all youkai species names are uncountable nouns, and therefore are written the same way in singular and in plural.