Biological evolution is one of the three most profound discoveries in science. The other two are the vastness of the universe and the depth of geologic time. As such, biological evolution, along with the other two, deserves to be the foundation of all science learning, and should be taught from an early age.
If the typical fourth grader can name planets in their order from the sun, she should also know that life is all related by descent from common ancestors. Life on Earth is One Big Extended Family.