Can a 21 yrs male have alzheimer's disease?

Rarely. There are some families with bad genes where several members develop alzheimer's disease at an early age but they are quite rate.
Yes. My first guess would be that this would be someone with early onset familial alzheimer's, but, yes, there are reports of onset in the teens and twenties. In the absence of a family history, i would want to know about metabolic or endocrine illness and HIV status, all of which can cause dementias that are indistinguishable from alzheimer's.