It seems like “adulthood” can be replaced with “adult”. But is “adulthood” used often in real conversation?
Native speaker’s answer
Rebecca
"Adult" and "adulthood" are actually not interchangeable with each other. "Adulthood" refers to the state of being fully grown or mature. In this video, the speaker says "only two of them would reach adulthood" to mean that only two of them would survive until they are fully grown. "Adult" refers to an human individual or animal that is fully developed or grown, and not the "state" of being fully-grown itself, like "adulthood" does. Ex: Most dogs reach adulthood at one year of age. Ex: All adults should vote.