Can a person have Huntington's Disease without showing any symptoms?

Yes you can have chlamydia for years without showing any symptoms.

The symptoms of Huntingtons Disease are, mental deterioration and uncontrollable movements; symptoms usually appear in middle ages.

No, there is no cure only treatment to try and reduce the effects of it.

Clumsiness Jaw clenching Loss of coordination and balance Slurred speech Swallowing and/or eating difficulty Uncontrolled continual muscular contractions Walking difficulty, stumbling, falling

If the turtles you're looking at are sick, don't buy them. It's often very difficult to cure diseases of captive reptiles, especially young ones, and also to get them to feed again if they do...