In the episode ‘Samantha’s Old Man’, Endora transformed Darren into an old man. This set up the situation for Samantha (Elizabeth Montgomery) to undergo two age transformations.
First the older Darren pretended to be called Grover and Samantha transformed herself to become Grover’s wife: this was achieved by simply replacing Elizabeth with an older actress.
Later Samantha transformed herself into an aged version of Samantha and refused to change back until Endora restored Darren’s youth. This transformation was achieved with makeup.
Rolf J Miller was nominated for an Emmy Award in 1971 for this makeup (Mission Impossible won).