Demented

[Sarah]

Entirely irrational: Completely unreasonable or lacking any sense of the consequences of actions taken.

Affected with dementia: Affected by the loss of intellectual functions that is associated with dementia. This usually only happens to elderly (old) people.

[Couch Potatoe]

Suffering from dementia.

[Marquetta]

The word means [mentally deranged], but so far nobody has mentioned how demented can be stretched, ironically, to include [clownish], [wacky] etc. Just the thing for the Urban Dictionary!

[Agnes]

unable to think or act clearly because you are extremely worried, angry, or excited by something: