Is it possible to paint Latex paint over oil base paint. It has been quite a few years since they were painted. Should a person sand the walls first then primer then paint?

First you will need to make sure it's an oil based paint. Take some acetone on a q-tip and rub the paint. If color comes off it's latex. Oil based paint will turn gummy.
If it's oil based lightly sand the walls and clean the dust off. I like using TSP.
Prime the walls with water-based Kilz-It tinted to your paint color. Let dry and you are ready to paint.

You can almost always paint latex over oil-based, and hardly ever get oil-based to perform over latex. If walls are texture sanding becomes almost impossible because of the rough surfaces. You can use a standard water based PVA primer as Patty stated and paint it with latex after it dries.

Hi, shablue55!

Just to reiterate here and support the other answers.

Are the surfaces that are to be painted perfectly flat, please?

Thanks!