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Old age decreases the elasticity of the arteries which results in increased peripheral resistance. Increased peripheral resistance decreases the cardiac output and increases the blood pressure.
Measure your blood pressure. If it is high treat it with salt restriction and appropriate medications.
It is due to aging process.
Cardiac output = Pressure/Resistance.
So cardiac output is inversely proportional to arterial resistance.
So the output would be low if the resistance is high.