adulthood
noun [ U ] uk/ˈæd.ʌlt.hʊd/ /əˈdʌlt.hʊd/ us/ˈæd.ʌlt.hʊd/ /əˈdʌlt.hʊd/
成年（时期）
the part of someone's life when they are an adult
People in England legally reach adulthood at 18. 在英国，年满18岁的人为法定成人。
Responsibility, I suppose, is what defines adulthood. 我想，成年就意味着责任。