Diagnosis for me took 9 months. I was 45.
As far as I know there is no one test to give a concrete diagnosis yet, I might be out of touch on that.
Often diagnosis for PD, and I dare say other conditions too, involves ruling out other things first, I was told this is because other conditions share similar looking symptoms.
The GP was my first stop, after his examination I asked him what he reckoned, he said PD but because of all the variables he wouldn't stake his reputation on it preferring the neurologist got to the bottom of things.
My symptoms were my gait had changed, left foot started dragging, upper body movements were slower prompting people to ask if I had a bad back or stiff neck, my left hand became very left handed for want of a better description, my sense of smell was on the blink and if stressed or going out doors on a cold day my left hand would start shaking.
BUT the mix of symptoms can vary greatly from person to person just like the mix, quantity and frequency of the meds we take post diagnosis.
Frustration is the order of the day.