We haven't done it yet, but plan to in the next 10 years or so. DH is thinking he'll retire in the next 2 years (at age 57), but hasn't definitely decided yet -- it will depend on how happy he is at his job. He originally planned to retire at 55 but is still enjoying his job, so not ready to take that leap. We have a 4200+ sq. ft. house (6 bedrooms, 4 full baths) and it's a lot to take care of. We also have 4 acres of land -- 3.5 of those acres is the yard that needs to be mowed and lots of landscaping that needs to be weeded. Four of our five kids are completely out on their own (and all but one of them live within 30 minutes of here) and the fifth one is a junior in college. We do not need this big of a house for the 2 of us. We do have the ability to live on one floor, so that's the only reason we'll be able to stay here for 10+ more years potentially. Honestly, if it was up to me, we'd be downsizing in 4 or 5 years. We did add a 400 sq. ft. room on the house a few years ago to be able to have all the kids and their families over at the house and all be in the same room together for a meal, so I guess we should try to get our money's worth out of that.

While there will be a little sadness on my part to leave this house (we built it), I could emotionally do it now. DH has a much stronger attachment to it and the stuff in it.
Ideally, when we downsize, I'd like to move to a retirement community where we can start out living in a 3-bedroom 2200-2500 sq. ft. house/cottage and then move to an apartment when/if we need to downsize more in the future or even move to various levels of nursing care if/when needed while staying in the same "community". Wherever we end up, we definitely want 3 bedrooms because we want to have a office/hobby room and a separate guest room.
As others have mentioned, we do not want our kids to have to deal with emptying and selling this house when we're gone -- or even having to take care of the house and the lawn for the months/years while going through that cleaning out/selling process. We're in the process of doing that with DH's parent's house right now and it has been a loooonnnggg, tedious, and stressful process. Our kids already have busy home/work/family lives, they don't need to spend months cleaning out our house and getting it on the market. IMO, it's our responsibility to take care of downsizing our house/belongings early enough in life so they have a much smaller job to do once we're gone.