You want them to share the same space, so that they can see/hear/smell each other, but not be able to peck. I do this for at least a week, but often more. Then you let them be together while supervised for a bit of time each day, and step in and separate if they fight hard--mild squabbles are...
This has not been my experience. I integrate birds of all ages all the time. You just have to let them get used to each other without being able to beat each other up for awhile.