Old enough to know better
Definition 1 of 1
Pro Tip 1/3Typical Usage
Used when someone old enough makes a mistake they shouldn't have, often because of immaturity. 
He's 30 and started a food fight, definitely old enough to know better.
Pro Tip 2/3Subtle Reproach
Implies disappointment without direct criticism; suggests the person is capable of wiser behavior. 
After losing her phone again, her mom said she was old enough to know better.
Pro Tip 3/3Expectation of Maturity
Reflects the expectation that age brings wisdom and fewer mistakes, especially obvious or silly ones. 
Leaving your homework until the last minute? You're old enough to know better.