If she has been in another game and she is a sheikah, then it is almost a certainty she is Impa. Perhaps she could even be the very same Impa from OOT. You might say that that could not happen because Skyward sword takes place too far before OOT for her to still be alive, but the actual time between the two games has not yet been revealed. Recently with the announcement that the temple of time has already been built and is in the game, the likelihood is that the time period between the two games is actually shorter than we first thought. We don't even know the lifespan of the people of Hyrule because nobody ever dies of old age that we see. We also don't know if sheikah have longer lifespans than regular Hylians. Kotake and Koume were 400 years old, and they did not look particularly bad for their age. I would say they looked about 80. If Impa also had this kind of lifespan then she could also look well for her age in OOT.