If I discovered that someone didn't know that the Sun is a star and believed it was a planet until they were 21, I would respond in a respectful and informative manner. I would try to provide them with the correct information and help fill in any gaps in their understanding. Here's an example response:
"It's interesting to learn that you believed the Sun was a planet until you were 21. The Sun is actually a star, and it plays a central role in our solar system. It's understandable how this misconception might occur, as the Sun appears significantly larger and brighter than other celestial objects we see in the sky. However, planets like Earth orbit the Sun, and the Sun itself is a massive ball of hot, glowing gas. It's a star of average size and is essential for providing heat, light, and energy to our planet. If you have any more questions about stars, the Sun, or anything else related to astronomy, feel free to ask!"