I have no hard data, but I have 10 years of anecdotal data (my wife was a 4th grade teacher for ten years and saw just about 1000 students over her career).
She says the younger kids definitely do worse by all measures in 4th grade, in large part because of their lack of maturity.
She says the younger kids definitely do worse by all measures in 4th grade, in large part because of their lack of maturity.