Marine turtles are illegal to buy for your aquarium. Marine turtles live over 100 years, but that is not true for fresh water/brackish turtles. Lifespan for them is about 25 years. Turtles are dirty. Many people I know who keep them run filters and also keep a hose in the tank that is constantly on with a drain on the other end so their water is constantly being changed.
Aaron, fresh water turtles under 4 inches cannot be sold (although they are all the time) because they carry salmonella.
In my avitar, you can see my Diamondback Terrapin, Dave. He is a brackish water turtle. Between the cost of lighting, filtration and maintenance, one or 2 of these guys will cost you as much as a reef.