If severe pulmonary hypertension is discovered in an elderly person then what, if anything, should be done to treat it? Should the condition be monitored and how would this be done?

Depends. There are several variables to consider and generally age alone isn't a reason not to be evaluated and treated for pah.
Certain diagnosis. If pah is suspected. A person has to be able to undergo the real test which is left and right heart catheterization or at least with proper exclusion of other possibilities. Next is to consider the treatments available and the diseases in the person. Is there contraindication to the medicines? If not then give the medicines, if approved by the insurance, including o2, all in all very expensive.