mid-15c., "peers collectively," from peer (n.) + -age. Probably on model of Old French parage. Meaning "rank or dignity of a peer" is from 1670s. In titles of books containing a history and genealogy of the peers, by 1709.
c. 1300, "an equal in rank, character, or status" (early 13c. in Anglo-Latin), from Anglo-French peir, Old French per (10c.), from Latin par "equal" (see par (n.)). Sense of "a nobleman of especial dignity" (late 14c.) is from Charlemagne's Twelve Peers in the old romances, who, like the Arthurian knights of the Round Table, originally were so called because all were equal. Sociological sense of "one of the same age group or social set" is from 1944. Peer review "evaluation of a scientific project by experts in the relevant field" is attested by 1970. Peer pressure is recorded by 1971.
word-forming element in nouns of act, process, function, condition, from Old French and French -age, from Late Latin -aticum "belonging to, related to," originally neuter adjectival suffix, from PIE *-at- (source of Latin -atus, past participle suffix of verbs of the first conjugation) + *-(i)ko-, secondary suffix forming adjectives (see -ic).