The most common and most likely answer to this crossword clue is the 9 letter word SENESCENT
S
E
N
E
S
C
E
N
T
More clues leading to the result SENESCENT
- Seen wandering by trail over the hill?
- Going down to get nurse for daughter, ageing
- Old nurse close to commode encountering aroma that’s characteristic of old age
- Seen to be dotty, having whiff, getting old
- Ecstatic about unfolding scene getting on
- Getting on a bit, primarily Neanderthal in views?
- Getting on, sent scene with alternative endings
- Dispatched outside — bad scene, being old
- Growing old makes sense for a change
- Growing old, making sense of change
Buddy explains!

The straight part
We know that "Growing old" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case growing old = senescent.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.