It means the same thing, and should probably be accepted, but stylistically, that would be «Mora til mora mi er gammel.», or «Moren til moren min er gammel.». Languages differ in the distinctions they make. Norwegian has distinct words for maternal versus paternal grandmother and grandfather, while English doesn't.
And what is the mother of a grandmother? Is that mormormor or mormorfar? If so it could also be morfarmor?