immature

An immature person or thing is not yet grown up or developed.

When we say that someone is being immature, it usually means they're acting like a child when they should know better. Maybe they're throwing tantrums, being overly sensitive, or acting selfishly. This can be frustrating for others around them because it's hard to have a conversation or make plans with someone who doesn't seem able to take responsibility or think things through. In general, we use the word "immature" to describe behavior that's not very grown-up or responsible.