My left bottom wisdom tooth is decayed and needs to be removed.. should the top one that is healthy be removed too?

Removed too: If you have a bottom tooth or top tooth removed, then the opposing wisdom tooth that would normally bite on it, is rendered useless and generally should be removed to prevent future infections or health-related complications. Ask your dentist for more information.

Yes: The maxillary tooth would have nothing to bite against (other than air and saliva), so it would not help you in eating. Over time, it would start to super-erupt, changing the contact with the adjacent tooth, resulting in food impaction. In the end, you're going to extract it anyway, so might as well do it now.

It depends: If the upper wisdom tooth has no caries or gum disease and still hits a lower molar, you should be able to retain it. If it does not hit a lower tooth, then extraction is usually recommended. However, at age 70 it is unlikely to supererupt enough to become a problem.