Each family is different and only the parent is in the best position to decide on a gradation date. That said, let's think through the pros and cons of graduating later than age 18.
The value of homeschooling high school is tremendous! We can be teaching age appropriate material that's consistent with your values. That doesn't mean that homeschooling should last forever, though. Eventually homeschooling ends, and we really SHOULD be sending our child into the world with confidence: Letting Go When Homeschooling Ends.
Why do you think your child should graduate late? There is no subject that's required to complete in order to graduate at 18. Read more about choosing a grade level: What Grade is My High School Child?
How does your child feel about graduating late? Consider also the friends of their age who are going off to college at age 18, which can cause some resentment even in the most sweet and compliant child.
Pros of graduating late.
One benefit of graduating late is that your child will be mature enough to handle community college in 11th grade and 12th grade. He or she can do dual enrollment in high school, earning college credits while in high school, and allowing them to quickly accelerate in math, foreign language, english, or science.
Dual enrollment is not always age appropriate and often not consistent with the our values. Dual enrollment goes 2-3 times faster than a high school class (making it hard to keep up) and, since the community college transcript will follow your child, the grades are forever and completely out of your control. That's why it makes sense to homeschool until you have an adult before jumping into community college classes. .
Cons of graduating late
It's nearly impossible to homeschool an 18 year old. If you decide to graduate your child late, plan on backing WAY off after age 18. That's very hard for a mom to do when a child is living in her home. Previously compliant teens can feel resentful about being sheltered well into adulthood, when they should be making their own decisions and learning independent living skills.
As you are planning the coming year? Don't just decide to graduate your child late or early until you have thought through all the issues. This may help you with your planning process: Planning High School Courses.