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! Do not touch that chicken! It's past its sell-by date, I'm just waiting till trash day to get rid of it. A: "Ew, that does not smell good at all." B: "Yeah, it's past the sell-by date."

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. After I work out now, I'm sore for days afterward, which is an unpleasant reminder that I'm past my sell-by date.

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.