Etymology
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New Age (adj.)
1971, in reference to a modern spiritual movement, from new + age (n.). It had been used at various times at least since the 1840s.
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middle age (n.)

"period between youth and old age," formerly generally understood as 40 to 50, late 14c., from middle (adj.) + age (n.). The adjective middle-aged "having lived to the middle of the ordinary human lifespan, neither old nor young" is by c. 1600.

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ice age (n.)
1855, from ice (n.) + age (n.). Perhaps translating German Eiszeit (1837). An earlier term in the same sense was glacial epoch (1841). Local scientific men had noticed from the late 18c. evidence that the Alpine glaciers once had been much larger; in the 1830s stray boulders, moraines, and polished bedrock in northern Europe (formerly interpreted as relics of catastrophic floods) began to be understood as revealing the former presence of a large ice cap there. When Agassiz, a convert to the theory, came to America in 1846 he found similar evidence in New England. The glacial theory and the notion that there had been several worldwide ice ages seems to have been generally accepted by the 1870s.
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Jazz Age 

1921; see jazz (n.); popularized 1922 in writings of F. Scott Fitzgerald; usually regarded as the years between the end of World War I (1918) and the Stock Market crash of 1929.

We are living in a jazz age of super-accentuated rhythm in all things; in a rhythm that (to "jazz" a word) is super-normal, a rhythm which is the back-flare from the rhythm of a super war. ["Jacobs' Band Monthly," Jan. 1921]
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Iron Age 
1590s, originally, as in Greek and Roman mythology, the last and worst age of the world; the archaeological sense of "period in which humans used iron tools and weapons" is from 1866 (earlier in this sense iron period, 1847).
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Middle Ages (n.)
"period between ancient and modern times" (formerly roughly 500-1500 C.E., now more usually 1000-1500), attested from 1610s, translating Latin medium aevum (compare German mittelalter, French moyen âge).
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Bat Mitzvah 

1941, literally "daughter of command;" a Jewish girl who has reached age 12, the age of religious majority. Extended to the ceremony held on occasion of this.

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sui juris 
1610s, "of full legal age and capacity," in ancient Rome, "of the status of one not subject to the patria potestas." For first element, see sui generis; for second element, see jurist.
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Agent Orange (n.)
powerful defoliant used by U.S. military in the Vietnam War, reported to have been used from 1961; so called from the color strip on the side of the container, which distinguished it from Agent Blue, Agent White, etc., other herbicides used by the U.S. military; see agent (n.). Banned from April 1970.
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double agent (n.)

"spy who works for two mutually hostile countries," but usually is loyal to only one of them, by 1920, from double (adj.) + agent (n.).

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