At what age does a dependent need to purchase his or her own plan?

Answer: 

Once you turn 26, you can no longer stay on your parent's health insurance plan.  At 26, you will need to get your own health insurance plan or you will pay a fee (https://www.healthcare.gov/fees/fee-for-not-being-covered/) when you file your federal tax return, which may be more money than you would have spent on a premium.

If you have insurance coverage from school or work, then nothing will change - you're fine. But, if you're on your parent's plan, you lose your coverage on your 26th birthday.

When you turn 26, you qualify for a 60-day special enrollment period and can sign-up for your own insurance. If you don't sign up within 60 days, you may not be able to get coverage until the next open enrollment period, and you may be fined. Depending on your income, you may be eligible for premium tax credits, making health insurance more affordable for you.

To learn more, visit: https://www.healthcare.gov/young-adults/