the period of life between youth and old age; midlife

(historical) A historical period of great accomplishment in a field or a society, usually following, and not quite as favorable as, a golden age, and not unfavorable as an iron age.

(idiomatic) A period of happiness, peace, prosperity, and/or great progress. / (mythology) The Golden Age.

The era, starting in the mid-1940s, during which nuclear energy was developed, and in which it was thought that all energy would be provided by such sources.

of, or relating to middle age; neither old nor young

The chronological age of individuals of the same maturity as the person in question.

(idiomatic) To reach a specific age where one is legally considered to be an adult. / (idiomatic) To mature, or become fully developed.

The period from the about the time of the launch of Sputnik in 1957 to the present day, during which people ventured into space. / (attributive) Redolent of the space age; appearing futuristic or modern.

(idiomatic) A time period of years or more.

Being of an age during which attendance at school is customary.