along in years

Also, on in years. Elderly, old. For example, Grandma's along in years now and doesn't hear too well, or Our dog is not as frisky now that he's getting on in years. This idiom transfers the length of along (and the “onward” of on) to the passage of time. [Late 1800s]

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.