I think a lawyer would have to weigh in on my comment to be sure.But I believe any person over the age of 18 has the right to sign legal contracts *unless* he/she has been declared "incompetent" by a court.And if I'm basically correct on this my guess would be that it's pretty much the same in all 50 states.
There are elder abuse laws in many states now that do not require the person be declared incompetent. These laws probably came about because of the increasing abuse of the elderly in recent times.