Is It Possible To Have Alzheimer's Disease At The Age Of 21?
Almost impossible.
Detailed Answer:
I read your question carefully and I understand the possibility of having Alzheimer's seem daunting to you.
I don't believe that to be the case though, that age is too early to have Alzheimer's. Over 90% of the cases with Alzheimer's occur after the age of 60. Even that remaining early onset group occurs later than your age, after 30 years of age. There have been cases as early as 15, but apart from being exceptionally rare, they were familial cases, inherited.
So, while there is not an arbitrarily defined lowest age limit, I don't think you should worry about Alzheimer's.
If you feel that you are having memory issues, I would like to make present to you that often what seem like memory issues are in reality difficulty to retain information due to concentration issues, at times relates to stress and anxiety.
I remain at your disposal for further question.