How to prep walls to paint after 55 years of applications?

For sure you will need a good cover paint to prime it. We used odor free Kilz paint to cover ours. There may be store brands that cover as well, but we wanted the best so we knew Kilz would do it. After the Kilz primer dries over night, paint as you need and like.

Thanks, I've heard of the brand for covering stains!

Don't worry about how many layers of paint the walls have. Just get down to the first layer that is not peeling. Use a dry scrub brush to clean away any peeling paint on the ceiling. Then paint with a primer, or a paint-with-primer. As long as you are sticking to the same or similar color, two coats will do it. You can use a third coat as a sealer, and you shouldn't have to paint again for a few years unless you have water damage. Best wishes 😇

If the ceiling is popcorn, they make repair patches and Homex has spray popcorn repair texture in cans.

Is there chance any of the paint is lead based?

Thanks for the responses! There's no lead, so that's a relief. The flaky ceilings are swirled, possibly with a sponge. The living room and master bath are a soft sage, but the rest of the rooms are white. The feedback has eased my concerns, and I'll start the project soon. Thank you!