It varies. The brain function start to decline as early as age of 30. The earliest development of alzheimer's disease could begin at age of 40. It can be last anywhere from 2-20 years with average of 8-9 years.
Alzheimers. Unfortunately there isn't an "age" per se for developing alzheimers. While most patients are elderly, alzheimer's does affect younger patients. Generally the patient is unaware of loss of cognitive function whereas the family is very aware.