The turtles in our local parks are more than likely the same species, the red-eared terrapin, as those little guys everyone kept as a kid.
Those kept as pets usually never reach maturity and die while still young, due to poor environment and nutrition.
The ones in your local pond are full-grown adults, who may live as long as fifty to seventy-five years, and who may have been pets that people set free decades ago.
When salmonella was recognized as a significant threat to pet terrapin owners, the U.S. Food and Drug Administration banned the sale of most pet turtles in 1975.
Before that, though, you could buy baby turtles in pet stores, department stores, and even drugstores.