Does "grown-ups" mean "adults?" Is it a commonly used term? Is it very informal way of speech?

Native speaker’s answer
Rebecca
They have the same meaning. The only difference is who you are talking to. The word 'grown-up' is mainly used when adults are talking to children, or when small children are talking among themselves. Ex: Let's pack our toys quickly before the grown ups arrive. Ex: You need to be respectful when speaking with grown ups.