You'd think the answer to the start date for college prep would be "around grade 11", however a better and more accurate answer is Pre-K. The reason is that today's college admission teams look at more than GPA and test scores.
Think about how early young children actually learn values and skills. Those values and skills learned in elementary and middle school are central to high school success. Students who work well with others, volunteer, step up to lead, who are assertive and also good followers, tend to look better on college applications later.