middle age
noun [ U ] uk/ˌmɪd.əl ˈeɪdʒ/ us/ˌmɪd.əl ˈeɪdʒ/
中年（通常指45岁至60岁之间）
the period of your life, usually considered to be from about 45 to 60 years old, when you are no longer young, but are not yet old
Once you reach middle age, you have to be sensible about your health. 一旦步入中年，你必须明智地对待自己的健康。