ripe old age
An age advanced in years, as in I expect to live to a ripe old age. The adjective ripe here means “fully developed physically and mentally,” but the current use of the idiom usually just signifies a long lifespan. [Second half of 1300s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.