c.1300, "made of gold," from gold + -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 (e.g. wooden, leaden, waxen, olden); Old English also had silfren "made of silver," stænen "made of stone."
As a color from late 14c. Figurative sense of "excellent, precious, best" is from late 14c. 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 from 1961. The moralistic golden rule earlier was the golden law, so called from 1670s.