ˌmiddle ˈage noun [uncountable]
the 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.

[TahlilGaran] Dictionary of Contemporary English ▲

middle age
noun
ADJ. early | late a woman of late middle age
comfortable He settled into comfortable middle age.
VERB + MIDDLE AGE approach a bald man approaching middle age
reach
PREP. in ~ He mellowed in middle age.
PHRASES well into middle age She had her first child well into middle age.

[TahlilGaran] Collocations Dictionary ▲

middle age

early 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 age
approach middle age (=be almost middle-aged)
a stocky, balding man who was approaching middle age
reach 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.

[TahlilGaran] Collocations Dictionary ▲

middle age
noun
BAD: Sitting next to me was a middle age man, probably about forty.
GOOD: Sitting next to me was a middle-aged man, probably about forty.
BAD: The Rolling Stones are all middle-age now.
GOOD: The Rolling Stones are all middle-aged now.

Usage Note:
The adjective is middle-aged (WITH -d ): 'They would prefer a middle-aged woman to a young girl.'

[TahlilGaran] Dictionary of Common Errors ▲