Longman Dictionary of Contemporary English
Douglas Harper's Etymology Dictionary
"period between youth and old age," late 14c.; middle-aged (adj.) first recorded c.1600.\n
Wiktionary
n. the period of life between youth and old age; midlife
WordNet
n. the time of life between youth and old age (e.g., between 40 and 60 years of age)
Wikipedia
Middle age is the period of age beyond young adulthood but before the onset of old age. Various attempts have been made to define this age and it can vary between cultures and historic or previous definitions of this stage of life.
Usage examples of "middle age".
Washington Hawkins had scarcely more than entered upon that decade which carries one to the full blossom of manhood which we term the beginning: of middle age, and yet a brief sojourn at the capital of the nation had made him old.
Napier looked good now that he was deeper into middle age-more imposing.