(My dictionary says relating to the senescence of an organism, but it comes to the same thing.)
But, look, I know there are plenty of crotchety old organisms out there, but gerontic is a horrible word to describe them, a awkward arthritic limp of a word without the slightest respect or compassion in it.
Old Man in Sorrow (on the Threshold of Eternity) by Vincent Van Gogh.
Let's be more tender to old age. After all, we'll get there ourselves before too long.
If we're lucky.
Sunday Rest: gerontics. This word comes from the Greek gerōn, old man.