Size of computer table. If your computer has a mouse on the right side of the computer, then the person on the left or in back is physically unable to get control without playing musical chairs. A trackball can be passed to all students. There's a guide to coop learning with computers in the Teaching Tips section of my website. Younger children can't do worksheets, thus, this idea may not work for them.
Every class has a personality.
Older kids seem to be more aware of each other at the computer. Older kids are more vocal about their needs and thus can exert more peer pressure to share. Younger kids may view using the controls as the whole point of learning with the computer, whereas older kids can focus on the content as well as the tool. The use of worksheets helps this happen. Since younger children can't read, worksheets don't work.