My father's 69 years old and in the past year his reaction speed to questions and his movement has slowed noticeably. is this an early symptom?

Possible, but relax: Early diagnosis of dementias are being encouraged, but they are subtle and need experts. He might visit a Specialist to establish a base-line of function and seek advice. Don't worry yet and don't upset him about it. Parkinson's needs to be ruled out too, so seek a good eval. You may be doing a very good thing in caring for him early. Thanks.