Hi A. Clark,
Zyrtec is well tolerated by most children and adults. In clinical trials 14 percent of adults have side effects of drowsiness and fatigue. People over 65 years of age did not have an increase in side effects according to clinical trials in the package insert. If kidney function is abnormal the dose needs to be lowered. Some elderly people may have higher risk of side effects because of poor kidney function.
The drowsiness and fatigue which may occur in a small percentage of patients may be more of a problem for the elderly because of the increased risk of bone fracture if they fall. Other medical problems common in elderly patients may contribute to sedation. Drug interactions pose further risk of side effects in those over 65 years of age.
I recommend Zyrtec to some of my allergy patients over 65 years but with caution regarding the above points.