Even accepting the premise that this is an indication of having a distinct personality, I don't think that's an adequate basis to afford infants personhood. Cats have distinct personalities as well, although this fact suggests that we could really use a better word than "personality." In fact, while there might be counterexamples that are not coming to mind, I'm inclined to suspect that every properly functioning vertebrate organism, as well as many invertebrates, has a distinct personality, albeit not necessarily one recognizable to humans.