"What grade are you in?"

My kids are frequently asked,

"What grade are you in?"

That is such a tough question for students that are homeschooled. We fit their education to their interests and their skills, not their age. At various points in their education they are "above grade level" in certain subjects, "below grade level" in others and very rarely, right "at grade level." Here is some info about homeschool grade levels. This year the girls will be studying Biology and Algebra together. The Engergizer will probably learning some of the Biology and Algebra too by overhearing and joining in on discussions. They take classes that are open to homeschoolers of all ages.
The Engergizer and Animal Lover are pictured in a science class that included kids from ages 6-15.

So how do we answer the grade question? We could choose to just answer with the grade they are expected to be in based on their age, but it just does not seem right, so they just say,

"We homeschool, so we are in different grades in different subjects."

What do you do when you get that question?