It is definitely NOT 18 inches AFF (above finsihed floor).
In NEC, there are requirements and then there are exceptions. There is a requirement for maximum mounitng height of 78 inches for controls/breakers/disconnects etc. I beleive the height is measured to the center of the operating handle from the floor. (I would tend to measure to the top of the unit, to play safe). You can mount a disconncet higher, if you equip it with a pull chain/rod such that the chain handle is within 78 inches AFF.
The most common practice is to install the disconnects between 54 and 78 inches AFF.
By the way the 18 inches is recommended hight (measured to the top) for mountnig convenience receptacles. ADA (in the USA) requiers mounting of receptacles between 16 and 48 inches, among many other things.
The motor disconncets have nothing to do with ADA.