Has anyone put a safety gate to prevent a dementia adult from going down stairs?

If you think this is going to be long term a door can be installed and removed later. The other suggestion would be to move the bed downstairs so that he does not have to do them at all. Much safer and he will be used to the bed in a new location when it becomes more and more difficult for him to go up stairs or when he is no longer able to do stairs at all. Sooner or later all care and transferring will have to be done, bed to chair and stairs will become impossible. Save the money on the door and get a downstairs bathroom more accessible if you don't have an accessible bathroom now.

I wouldn't count on a gate to deter an adult. My late husband broke his fibia when he leapt over the guard rail of his hospital bed. He fractured his fibia, and Four months later he died of the fall that resulted from this incident. Try to find a sturdier solution,

We had a gate installed at the top of the stairs. It was anchored to the wall but was a difficult to open gate. It served its purpose and if had been for the long term would have had something sturdier installed. I was not confident that the gate would have held up to a fall for trying to force it open. Damage to the wall from mounting can be fixed and was the least of my concerns.

Look for a Gatekeepers dealer near you, they make matching wood/paint/stain heavy duty gates that are high enough and strong enough to withstand an adult's weight.

A lady I cleaned for had a half door installed across the basement stairway, she said her husband would stand there shaking and pulling it to try and get downstairs, so I would be apprehensive putting up anything that isn't very sturdy. I have read that sometimes a black mat placed in front of a doorway will keep them from crossing it because they perceive it to be a hole in the ground, I don't know anyone who has tried it though. There are probably some good ideas using technology... perhaps some type of motion sensor alarm when he approaches off limit areas?