Another word for showing adult behavior?

Here are some words that can describe showing adult behavior, depending on the specific nuance you want to convey:

General Maturity:

* Mature

* Responsible

* Sensible

* Level-headed

* Self-sufficient

* Well-adjusted

* Sophisticated

* Discretionary

Specific Behaviors:

* Thoughtful

* Considerate

* Empathetic

* Respectful

* Self-controlled

* Disciplined

* Reliable

* Trustworthy

* Independent

* Self-aware

* Accountable

More Formal:

* Decorous

* Protocol-bound

* Composed

* Gracious

Informal:

* Grown-up

* Matured

* Wise

The best word to use will depend on the specific context.