We have house built approximately 1944 with plaster and lathe interior. Above every door and window there are diagonal cracks from 1/8 to 1/4" wide. I would like to know if this is because of the house shifting or because it is old plaster. Thank you

Yes, it is.

Cracks like that are very common in sheetrock as well as lath and plaster walls as a house settles. They make flexible material specifically for patching them that helps reduce the length of time between repairs.

To expand on what LA Marlowe has said, the corners of door and window openings are places where stresses are concentrated, and the stresses come about because of natural expansion and contraction of your framimg, and shrinkage of the lumber itself. It doesn't take much movement to produce those cracks, and they do not necessarily indicate any serious structural problem.

Cracks are indicators of were movement is taking place on the wall. AS a general rule If you follow the crack towards the floor at a right angle to the crack you will find the area in which the floor has settled. The corners of the door are the weak point on the wall so the tear starts there first.
If you close the door you also can see the settlement. Facing the door from the inside, if the space at the top is tigher on the door knob side the wall as settled down on that side. if the same side has a gap the wall has settled down on the hinge side. the crack should be on the side that settled.
lIke Richard said. This is not normally a great concern other then the repair being a pain, but if you had recently performed any major framing change below the rooms with the door cracks, You need to look into that further.
Rmember the wood in a house shrinks with age. more so on the width due to cell shrinkage. Homes are built so fast these days the lumber never has a chance to properly dry out. Thus the resulting shrinkage. Quite often builders offer a door adjustment after one year because of this. Older homes are much worse as they did not have forced drying kilns and the wood was often cut down on site and then comverted to lumber and put up in the form of a home. I have seen settlement so bad that the owners of the home had shims under the toilet because the framing dropped and the cast iron plumbing never moved.