You have received good advice from the other two posts to date. They can leak at any time depending upon when they rust out, etc.
Have them inspected now. Contact the city and state agency that deals with it such as water quality, health and environment, etc. Chances are they have to dig to inspect them so unused tanks should be removed. They usually remove the tank and any surrounding contaminated soil if it leaked. If it leaked they could require monitoring wells drilled around the neighborhood like they do with gas stations (very expensive).
The EPA and the state can also go back against former owners for spills, etc. but that is after much litigation and much of your legal fees. I personally wouldn't buy a place with an underground tank but do what you want to do. There are land deals going on where there used to be a gas station many years ago that no one knows about until after the ground water is contaminated. Then someone is stuck spending mucho bucks.
Consult a lawyer in your area about this asap. Cost sharing could be you paying anywhere from $0 to all depending upon what you negotiate. I would try to have them pay it all. You could also consider a purchase clause that it is conditional upon removal and clearance given by the state and/or the EPA although their clearance doesn't automatically clear prior landowners. Also include a warranty in the deed that specifically binds them to being responsible if it has a leak.
I'm not sure if it is used, if one is needed, if there is an alternative site such as a basement, or what. Please post back with that information. Chances are that if it has some age, then it should be removed or replaced (and at their expense if you can negotiate it). Gas stations everywhere are dealing with this very problem and some are closing rather than spend the money for new tanks which also have new requirements for placement in the ground to protect the ground water. Good luck.