According to Healthcare.gov

If a plan covers children, they can be added to or kept on a parent's health insurance policy until they turn 26 years old.

Children can join or remain on a parent's plan even if they are:

married
not living with their parents
attending school
not financially dependent on their parents
eligible to enroll in their employer's plan

These rules apply to both job-based plans and individual plans you buy yourself, inside or outside the Marketplace.