What does Jonas learn about what happens to the old in The Giver?
There are a lot of things that Jonas learns about the old. For example, he learns that they are hit with discipline wands when they misbehave, just like kids are. But I think that you are talking about the fact that they get released when they are deemed to have lived out their usefulness. Jonas learns this relatively early on when he is told about a Release ceremony that has happened at the House of the Old. Later on, after he has become the next Receiver, he learns that "release" means killing.
So Jonas learns over the course of the book that old people get killed when they are no longer deemed useful to society.