Seems to be 2 Impa-threads^^;;
But considering this one were bigger im going to copy what i wrote in the other thread.
The SS-impa dies in SS, clearly, but there were other Sheikahs. (otherwise the whole race would have died out, there were actually many Sheikahs during the Hyrulian Civil War before OoT) So that makes OoT-Impa an decendant.
Impaz who is found in old Kakariko village (hidden village=kakariko from OoT) in TP is the child of OoT-Impa. No doubts about it. I think the OoT-impa might have died when the Gerudo invaded Hyrule between OoT and TP. Explaining why she isnt around in TP.
However, in the official "alternate timeline" where Link fails, OoT-impa survives to become the old woman Impa in the original LoZ and AoL. She's able to survive that long because her already extra strong magic abilities got enhanced by her position as a sage. Powers she didnt pocess in the child timeline