I think the average elderly person who has a cluttered home isn't necessarily a hoarder. I think it is more to do with life cycles and "nest" building.
To generalise massively , we get married and have a relatively clutter free existence before DC. We are full of oestrogen. We build a family and even a pair of bootees takes on immense significance, and then all the toys and clothing and "stuff" enters the house. And it's at that point we accumulate proper furniture and accumulate things that are about material comfort more than absolute necessity, such as framed pictures and maybe "best' dinner plates and glasses and extra sets of bed linen.
Then you reach "peak possession" when the DC are teens, our oestrogen falls away, and as they start moving out, we have more time, we declutter and downsize and get rid. Except some people don't. And then they get old or ill and can't. And it all runs away with them.
But this pattern is changing I think. For a start, steady jobs and homes, are a lot harder to come by and we move more. Property is vastly more expensive. More people rent. And rent for longer, Houses are smaller. Fewer people live in the same family house for twenty years. Hence the need to have a more minimalist existence which can be packed up and moved easily. Decorating trends tend to mirror economic peaks and troughs.
And in our parents time, possessions such as furniture, weren't as easily available as they are now. You had to save up and there wasn't a great deal of choice. Brown furniture was heavy and solid. Now you can go to IKEA and it's all there in one shop. Easy to put up, and take down, so they view it as more disposable I think! Hence younger people favouring a much less personal, uncluttered, minimalist space.
Personally, I think it's essential to declutter mahoosively before every house move and contrary to what a pp said, never, ever take on a storage unit unless it's for a few weeks to facilitate a move. There are storage units up and down the country rammed full of possessions that no one wants. When it's out of site it's out of mind. And that modest monthly fee adds up. My sil has spent £18,000 storing furniture over 20 years that she has barely visited in that time. Don't do it!
Having observed a few extended family members with hoarding issues I think a lot of people with excess possessions are unable to face themselves as they are now, they are hiding from reality and harking back perhaps to more comfortable times (childhood?) when other people were in charge and made all the difficult life decisions on their behalf, it's an avoidance technique to face the reality of the here and now, a sort of arrested development. Maybe?