Start by opening an image of a young lady with face close up.
Insert another image of an old woman with the same face angle.
Reduce the opacity to see the below layer. Scale down and place the old woman's face according to the face below.
Click add vector mask button from the bottom of the layer palette.
Select a black to white gradient in linear mode. Apply the gradient from left to right.
Select black as a foreground color. Select the brush tool. and apply it to remove unwanted areas.
Change the layer mode to luminosity.