Their accession to human status is stopped by only the unwillingness and refusal of other human beings. They can fulfil all the criteria it takes to be human, but they cannot force an acknowledgment from those around them. We have one of the nicer people, J.F. Sebastian, who suffers from a premature aging disease, Methuselah syndrome. There is also the statement that the replicant Rachel "won't live, but then again who does?" The length of one's life is not what makes us human.
Rachel is the replicant who believes she is human, because she can remember her past. Unfortunately Deckard is quick to tell her that her memories are implants and belonged to the niece of her creator, Tyrell. Memories then can be faked. They do not last as Roy points out in his moving final words "all those moments will be lost in time, like tears in rain." Our memories are not what make us human.
We see another possible answer in Roy's journey to the "Tyrell Corporation" building. It has been suggested that this looks like a pyramid or a temple - the place where the creator ("god") of the replicants lives. Tyrell is at the top of this pyramid, which suggests he is the most important person, so he is the creator. Roy goes there and asks the same questions that humans have longed to ask God. Why did you create me? Why did you design my life to be so brief? Can you not make things better? Roy has reached the point in development where he struggles with existential issues just like humans. Tyrell's answers do not provide Roy with any hope or satisfaction, so Roy kills his "god". Our gods and religions are not what make us human.
By this time, Deckard had "retired" all of Roy's companions. It seemed Roy would have to face imminent death alone. In the final fight scene between Roy and Deckard, Roy's intentions are to get revenge for his friends" deaths. In the climatic moment of this scene Roy seems to accept his fate. He discovers an appreciation for life that goes beyond his basic instinct for staying alive.
