Somebody’s declining years
Definition 1 of 1
Pro Tip 1/3Respectful Tone
Used carefully to show respect when speaking about someone's later life. 
The author discusses the scientist's discoveries made in his declining years.
Pro Tip 2/3Not Always Negative
It doesn’t always imply poor health or sadness; can be neutral or positive. 
In her declining years, she took up painting and created beautiful artwork.
Pro Tip 3/3Emotional Context
Can evoke sympathy or reflection on mortality and the passage of time. 
His music gained depth and emotion in his declining years.