How early to start working on college credits?

[AJ_Atlanta]

A high schooler who does well in school should take CLEP U.S. History 1 and 2 as soon as those high school history courses are finished. Take the exams while the material is fresh. The reality is that academic core courses or general education courses or whatever you want to call them are not the courses you want to take in college. They are mostly weed out courses. Not taking these courses in college isn't missing out on anything.

Actually I think a HS should take the CELP general test at the end of school, they are so broad it's not something you can study for 10-20 years later

I would highly discourage anything other than AP and courses at the community college. Even CLEP is risky, but far less risky than ACE. I certainly would not spend money on SL, study.com, shmoop, saylor, etc. because they will be worthless if the child wants to go traditional. If the student is insistent on going to the big-3 or WGU, then my answer above doesn't apply. Nontrad is better for those who are already working. I'm not sold on nontrad being ideal for teens.

For AP and community college, start them early as long as they want to do it and are able to do it. 10th grade would be great if it is possible.

I think every traditional school I have ever looked at accepts CLEPs for incoming students, limited usually to 12-18 credits worth but still widely accepted. Granted if your school has an AP program or a dual enrollment you should take advantage of it.