When was the planet Jupiter declared a planet?

Answer 1

Galileo's time or possibly earlier. Planet originally meant "wanderer" in greek because of how they moved across the sky. A number of asteroids were called planets at one time because of what the word planet meant back then.

Answer 2

Jupiter has been known as a planet throughout human history due to its visibility to the naked eye. It was officially recognized as a planet in the modern sense when the heliocentric model of the solar system was accepted in the 17th century.