/\/\/\/\/\ What she said. Same thing happened to a friends son a few years back. Sole of the boot came off the boot and stayed in the ski. Fortunately no one was hurt, but since the sole of the boot stayed in the ski, the brake didn't deploy and kept going down hill at speed. It could have ended much worse.
All boots, old and new are subjected to temperature cycling; hot, cold, hot, cold etc. This stresses them. Today's plastics can be stronger and more durable, but they still get stressed. If the boots are 20 years old, with all that is available to you now, you owe it to yourself to get a newer, better, safer pair.
Skis and bindings are available in abundance and since you are getting back into it, before buying anything, you might want to take a refresher lesson and get used to the new gear. It's a little different.