Turtles are actually pretty high maintenance, they require expensive tanks and all will grow to the size of a dinner plate.
You may seem them advertised as minature, etc. but there is no such thing as penny turtles. On top of that they require lighting and special diets. They're a pretty hefty commitment, its not unusual for them to live 70 years or more.
And they aren't easy to rehome when they aren't little and cute anymore.