If you know what you're doing, yes. I do ground control and showmanship training with my soon-to-be yearling. I'm prepping her for in-hand trail and yearling longeline later in 2013.
However, I do not recommend working your yearling hard unless you know what you're doing. Like everyone said, you can put a lot of stress on their bodies if you are not clear and direct from the get-go. You set your yearling up for success every time, and make their successes as quick as possible.