My Mum was diagnosed at 69, weeks after being recommended intense chemo to zap any cancerous lymph nodes by an enthusiastic consultant at the Royal Marsden; on a private note the GP said it 'probably' caused her Parkys.
She died aged 84, the last few years of her life non-speaking and in various care homes but very much 'on it' we took her to see the last Bond film at the time and she enjoyed it.
So how individuals react to the drugs is very variable and some of it depends on whether the agonists work or drive them mad with hallucinations and so on.
Also, do from time to time check they really do have Parkinson's, it's an annoyance that I do wonder if the drugs didn't bring it on more, or whether Mum had something like 'restless arm syndrome' or something.
Most important advice - ffs get your dad to grant you and his other sons and daughters and wife Lasting Power of Attorney in Health and Welfare while you still can. You cannot get it when he is deemed to have lost mental capacity and the bar is set low for that - and in any case, if he is out of it in temporarily in hospital, that amounts to the same thing anyway. Without LPA in Health and Welfare, you will not be the decision maker for his care, the State is - and the State profits from your parent's death, to put it brutally and bluntly.
Maybe get a bone scan test, if they are weak a fall can lead to a breakage. Powder laxatives because constipation is an issue. Use a Zimmer with wheels to go up the road - I used to tug my mother along the road, she was very much up for it, but a row with my Dad made me go non contact and that was it really, no one else did that with her... This was some years after diagnosis. Make sure he gets proper hydration, oh and the piss pot or commode by his bed for nights - an empty beetroot jar is good, it's the fall that gets you.
Be sparing about calling the ambulance out - my Dad did it when really he just needed to get her up off the floor, after a couple of times your card will be marked and they will keep your parent in hospital to manoeuvre for them to go into a home, to get you on the conveyer belt... That's about £2K a week.
We got Mum out to Paris for the first time twice after her diagnosis, she went on the Eurostar, once the drugs settle there is lots of stuff to enjoy. I may post a few photos later on...