At our very good local public school, the kids start learning very superficially in kindergarten about fractions. My second-grader (who tested into the 5th grade as a first grader for math on an achievement test) is "learning" about more complicated fractions.
I remember learning about fractions in 5th grade, but they are definitely learning alot now in 2nd grade. I think in third grade they will start adding fractions with different denominators.