Brief Answer:
Probably Parkinson's Disease.

Detailed Answer:
Hello dear,
I had gone through your question and understand your concerns.

There are many reasons that can cause involuntary shaking of the head. However, considering the age, I think that it can be a Parkinson's Disease.

Parkinson's Disease is a very common cause of this symptom in this age.
You should consult a neurologist for a confirmation of this diagnosis.

If the diagnosis is confirmed there are prescribed medications (Dopaminergic drugs) that maintain under control this disease. Please consult your doctor about this treatment.

Hope my answer was helpful.
If you have any other question I'll be glad to help.

Regards,     
Dr.Dorina Gurabardhi
General and Family Physician

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