Is severe pulmonary hypertension generally considered to be life-threatening in a 90 year old, especially if it is untreated?

Yes. It is life-threatening at any age, and more so, it simply decreases the quality of life for inability to do much activity and be able to oxygenate adequately.
It can be life threa. It can be life threatening if not treated. The treatment depends on the cause and most may need oxygen if it is due to chronic lung disease and other appropriate treatment by your doctors. Those with primary pulmonary hypertension in which no cause is found it is usually treated with drugs there is no cure and if not treated and severe it can be life threatening, especially at age of 90.