Mo. Rev. Stat. § 459.035: Declarant presumed to be competent.

For the purpose of sections 459.010 to 459.055, a physician or medical care facility may presume in the absence of actual notice to the contrary that an individual who executed a declaration was competent when it was executed. The fact of an individual having executed a declaration shall not be considered as an indication of a declarant's mental incapacity. Advanced age of itself shall not be a bar to a determination of capacity.