As you probably already know, self expression and putting thought into physical form are a large part of it. If the children are truly interested in the subject and tools then they will keep the play going for longer lengths of time. Having posted pictoral documentation as a reminder is a key idea for the children still in the process of learning to trace back to build on what they learned from their last experience with the same sensory items or manipulatives. For this reason it is always age appropriate. If they can think it and apply their ideas, then they are obviously old enough for it.
Environment also plays a large part in it.
Just this weekend I went to a magnificent workshop by Deb Curtis and Rhonda Iten on environments. Early Childhood Education at the University of Illinois.