We keep them until they can no longer be fixed. Our newest car is a 2009 Toyota bought three years ago. Cars are a waste of money in general and the transaction costs are annoying.
As cars have gotten more expensive and reliable, most Americans no longer buy new cars. A shrinking number of people are purchasing the cars that we all will be driving over the next 25 years. This has important implications for the size/safety of the cars and trucks that we will be driving in 2030 or 2040 or 2050. It also will drive the share of the market that will be highly fuel efficient or electrified.
Bottom line, it takes a LONG time to turn over the nation's vehicle fleet.
https://www.nytimes.com/interactive/2021/03/10/climate/electric-vehicle-fleet-turnover.html
Electric Cars Are Coming. How Long Until They Rule the Road?
In 2050, when electric vehicles are projected to make up 60 percent of new sales, the majority of vehicles on the road would still run on gasoline.
If the United States wanted to move to a fully electric fleet by 2050 — to meet President Biden’s goal of net zero emissions — then sales of gasoline-powered vehicles would likely have to end altogether by around 2035, a heavy lift.