From Longman Dictionary of Contemporary Englishmiddle ageˌmiddle ˈage ●●○ noun [uncountable] TMCAGEthe period of your life between the ages of about 40 and 60, when you are no longer young but are not yet old Men who smoke are more likely to have heart attacks in middle age.COLLOCATIONSadjectivesearly middle age (=around age 40)Two women in early middle age sat next to him.late middle age (=around age 60)a well-dressed man in late middle ageverbsapproach middle age (=be almost middle-aged)a stocky, balding man who was approaching middle agereach middle age (=be middle-aged)You need to start saving for retirement before you reach middle age.be well into middle age (=be obviously middle-aged, probably at least 50)Most of the people there were well into middle age.
Examples from the Corpus
middle age• It was a problem of youth, not of approaching middle age.• The Crown Prince had reached early middle age without marrying.• Memory of life in middle age tends to be sketchier.• In middle age a nation seeks safety and consolidation of material gain.• In middle age he has experienced a breakdown, an identity crisis, which followed a long illness and an operation.• The new technique allows women to have children well into middle age.• Life expectancy for all patients is middle age, and heart failure is the leading cause of death.• She was twenty-three and simply too young to comprehend the feelings of middle age - let alone those of a middle-aged Prince. 