In most of us, by the age of 30…
In most of us, by the age of 30, he writes, "the character has set like plaster, and will never soften again."
— from The Relationship With Yourself (Traits/Reflection) · Me, But Better: The Science and Promise of Personality Change by Olga Khazan
In the book
You are not stuck with the traits you happen to have. The old line — that by thirty the character has "set like plaster and will never soften again" — is simply not true. Changing a trait is possible; it works just like changing a habit. […] Talk to yourself like someone you love. Catch the inner bully in the act; aim for excellence but never let perfectionism stop you from trying; and refuse to let the perfect be the enemy of the good. Build your character on purpose. You are not set like plaster: pick one trait, set small goals, act the part, and track it in a log. Choose optimism over pessimism, humility over pride, grit over giving up, and generosity over entitlement. — The Relationship With Yourself (Traits/Reflection)