Creating a transcript is covered in the next section, here the focus is on gathering the information that will appear on the transcript. There is no legally-defined age when a student is considered to be in highschool. Even in public school, students skip grades or are held back. The only drawback to this would be that colleges will investigate more closely the transcript of a student applying for admission if he is underage. SAT scores that verify the student's academic preparedness will go a long way to proving the validity of your transcript.
Transcripts record only the final course grade earned by students.
Grading can be subjective or objective. Objective grading would be based on actual scores from daily problem sets or worksheets, quizzes, and tests. The type of curriculum or non-curriculum you use will have some bearing on how you will grade, if you choose to. These could all be averaged and the final figure become the student's grade. Unschoolers could simply grade the various projects their students do titling them general or specific subjects.