be past its sell-by date

be past the/(one's) sell-by date

1. Literally, to be past the date by which an edible item should be sold before it spoils. That milk is definitely past the sell-by date—it's curdled!
2. By extension, to be past the peak of one's abilities or career. The company is forcing my mother to retire because they think she's past her sell-by date. That player is definitely past his sell-by date—he hasn't had a good season in years.
See also: date, past

be past its ˈsell-by date

(informal) be no longer useful or valued: His ideas on economics are well past their sell-by date.
The sell-by date is the date written on food packaging after which the food must not be sold.
See also: date, past