Odds of having alzeihmers at 21?

Almost Nil: I am almost certain you do not have Alzheimer's disease. There are, however, a TON of reasons why you might have memory issues at 21, such as stress/anxiety/depression, vitamin deficiencies, hypothyroid, head trauma/concussions, and illicit drug or prescription drug side effects. If you are having significant problems with your memory, I would discuss it with your primary doc or a neurologist.

Almost zero: There is an early onset Alzheimer's but that onset is also much older. Alzheimer's is considered a disease of aging. Dementia does not mean alzheimer's. There are lots of different causes of dementia that can occur at different ages.