I'm 63, DH is 72.
We sold our 4 bed family home about 4 years ago, mainly because it was too big for us & had a lovely big garden that was becoming too much for us to manage.
We learned from my parents who moved from a 6 bed thatched cottage in the country with a huge garden & about 5 acres of paddocks into a 4 bed house in the city. My father badly didn't want to move, but they lived 'in the sticks' my father had Parkinsons & dementia & my mother had a stroke which meant she could no longer drive. Daddy made the move very difficult unpacking boxes that had been packed, arguing over every choice of house to move into. But they had no choice as living where they were was no longer tenable.
When my mother died we spent several weeks clearing my parent's house of 60 years of accumulated 'stuff' that they'd moved from their old house. Frankly it was a mess. They hadn't redecorated in the 12 years that they'd lived there, & after my father died, my mother wasn't motivated to clean. She would tell me off if I cleaned for her accusing me of passively aggressively criticising her for having a dirty messy house - which it was. But all I could do was remove the out of date stuff from the fridge & give the kitchen & bathrooms a once over when I was using them as it was her house &, she was right, I had no right to clean it.
DH & me decided that we didn't want our children to have the same job of clearing out a mess & down-sized.
So when we moved into our 2 bed bungalow we filled 2 skips with rubbish from garden sheds & the rooms & made more trips to the recycling centre & charity shops than I can count.
However, we're having a new kitchen fitted into our bungalow & have identified loads more stuff that we no longer need. I'm talking the family sized Le Creuset casserole, veg dishes, milk jugs, sugar pots etc etc. All going to charity. We've also sold a lot of my collection of Art Deco items & pictures inherited from my parents at auction & on-line websites.
You can't force your parents to do it - it has to come from them.