That is very possible.
Onset age is typically >60.
It is possible Biden has early symptoms of parkinsons, early enough they arent showing publicly. But if that is the case it would be likely unrelated to his dementia.
Also keep in mind there is a type of dementia called parkinsons dementia where he has the symptoms of both with a common underlying cause (lewy body dementia is common)... so if parkinsons is involved it is clearly alongside dementia.