1620s, "a critical stage in human life, a period supposed to be especially liable to remarkable change with regard to health, life or fortune," from Latin climactericus, from Greek klimakterikos "of a critical period," from klimakter "rung of a ladder" (see climax (n.)).
By some, held to be the years that are multiples of 7 (14, 21, 28, etc.), by others only the 3rd, 5th, 7th, and ninth periods of 7 years (21, 35, 49, etc.), to which some added the 81st year. By still others it was regarded as the years that were multiples of 9. The greator grand climacteric,supposed to be especially remarkable, was the 63rd year (7x9) or the 81st (9x9).
In 19c. medicine it often especially meant "menopause." Climacteric was used earlier in English as an adjective, "pertaining to a critical period or crisis" (c. 1600; climacterical in this sense is from 1580s).