When you already have an opinion about something before you've given it much thought, you can call that a preconceived idea. Many people have preconceived ideas about teenagers.
It's almost always an idea or opinion that's described as preconceived. You might have preconceived ideas about what college will be like — in other words, you've formed a strong sense of what the experience will be before knowing much about it. Most preconceived ideas don't come from strong evidence, and many aren't true. The adjective is formed from pre, "before," and conceived, which here means "taken into the mind," from the Latin concipere, "to take in and hold."