This is a difficult question to answer. There are several variables to consider. First, each college course is different. There are easy courses and very hard courses. The harder courses require more effort and education, whereas the easier courses require less. Second, the level of intelligence of each student differs. I know that this is a sensitive point, but some students are more intelligent than others.
With that said, it would never be a bad thing for a student to come into college with more advanced coursework. Some review in college can be a good thing. Moreover, if the student finds something easy, then there are always other courses they can take or even independent research projects. They sky is the limit at college.
As for minimums, a high school diploma or equivalent should be a basic standard. To go lower than this would be a students at a disadvantage.