Jump the shark
From The TV IV
Jumping the shark is a term used for when a TV show continues past its peak, often by having a near-to-impossible or unrealistic event occurring.
Today the term is used to refer to any series that is past its prime. Massive changes to the shows' cast or storyline are common markers for this decline. Television shows where the kids grow up and go to college, such as Boy Meets World can force the show to undergo many changes. Another possible marker is when main characters get married, like in Friends.