I would like to know when, at what stage, does alzheimer's starts to prejudice executive functions? how can we manage to enhance behavioral inhibition?

Not definitive: Alzheimer's and other dementing illnesses have no definitive time frame of when certain dementing "milestones" occur. Some patients may have minimal symptoms for years and others can rapidly progress to more serious "executive dysfunctions." the medications can "slow" the progression, but are not curative. I recommend seeing a neurologist who specializes in alzheimer's to get serial testing.

Can be early: Poor executive function(complex tasks) maybe one of the first signs of ad. Often going undetected, as the symproms maybe very subtle in early disease.A neuropsychological eval can be very helpful. Difficult behaviors are common in dementia. There are no drugs approved for mangement, but working with a practitioneer experienced in dementia care would be helpful.