c. 1300, "made of gold," from gold (n.) + -en (2); replacing Middle English gilden, from Old English gyldan. Gold is one of the few Modern English nouns that form adjectives meaning "made of ______" by adding -en (as in wooden, leaden, waxen, olden); those that survive often do so in specialized senses. Old English also had silfren "made of silver," stænen "made of stone," etc.
From late 14c. as "of the color of gold." Figurative sense of "excellent, precious, best, most valuable" is from late 14c.; that of "favorable, auspicious" is from c. 1600. Golden mean "avoidance of excess" translates Latin aurea mediocritas (Horace). Golden age "period of past perfection" is from 1550s, from a concept found in Greek and Latin writers; in sense of "old age" it is recorded from 1961. San Francisco Bay's entrance channel was called the Golden Gate by John C. Fremont (1866). The moralistic golden rule earlier was the golden law (1670s).
Whatsoever ye would that men should do to you, do ye even so to them [Matthew vii.12]
Do not do unto others as you would that they should do unto you. Their tastes may not be the same. [George Bernard Shaw, 1898]
ancient southern constellation, 1590s, from Latin āra "altar, hearth," from PIE root *as- "to burn, glow."