How big do you want them to be? They will branch at the nodes just below the cut. If full sized trees are your goal, wait until they get about 4 feet tall and cut off the top 12 inches. If you prune them now, you are creating bonsai.
Also remember that you may not get fruit like the parent tree. There's a lot of genetics that come into play with seeds. You will get your first fruit in about 3 years.
Daisy
