What is a definition of a mature team?

A mature team is a self-reliant team, self-regulating team. A team that has a robust mythology and rituals. They are capable to adapt to external factors easily, because every member knows (often subconsciously) how to act.

Does every team member needs to understand organisational theory? Not really. But they need to think in similar frameworks as other team members.

Is a mature team is an old team? In a sense. It takes time for members to adjust to common working. But the team does have to have a long history, or even have middle-aged members.

I think there's a correlation between member's professional and personal maturity and the team's overal maturity.