At what height should CO detectors be installed?
Carbon monoxide detectors should be installed according to the instructions that accompanied the unit.
From Nest Protect's website:

There’s a myth that all carbon monoxide alarms should be installed lower on the wall because carbon monoxide is heavier than air. In fact, carbon monoxide is slightly lighter than air and diffuses evenly throughout the room.
According to the carbon monoxide guidelines of the National Fire Protection Association (NFPA 720, 2005 edition), all carbon monoxide alarms “shall be centrally located outside of each separate sleeping area in the immediate vicinity of the bedrooms,” and each alarm “shall be located on the wall, ceiling or other location as specified in the installation instructions that accompany the unit.”
Standalone carbon monoxide alarms are often placed low on the wall, but it’s not because they’re more effective at that height. It’s usually because they need to be plugged into an outlet near the floor or have a digital readout that can be easily read.