The K12 program doesn't list a specific age limit. For age requirements you would have to check the website for the individual K12 partner school.
In The California Virtual Academies students must be at the age of 18 or younger upon enrollment. Students under the age of 14 should provide evidence of eighth grade completion. Some exemptions apply, including students who have been continuously enrolled in high school after the age of 18 making progress toward a diploma.
To find a K¹² partner school in your area visit K12 - Find a School and Enroll.
Find a K12 current high school course list here
Is online learning right for you? Try the K12 interactive questionnaire at K12.com.