Mammals have warm blood and hair on some or all of their bodies, and because a turtle lacks both of these things, it is not a mammal. A turtle is a reptile.
Animals in the reptile family include turtles, crocodiles, alligators, lizards and snakes. They have been around for at least the last 315 million years and have evolved little during that time.
There are currently around 400 species of turtles and tortoises in the world. In the reptile family, turtles are older than crocodiles and snakes.
Turtles cannot breathe underwater and spend most of their time on land. Although some turtles have soft shells, most species have a hard protective shell.