One is in a short story set in New York City by Truman Capote. The narrator decides he is all grown up because he realizes that not everyone loves him.
In 12-step programs you are grown up when you accept that you are powerless over the whatever - booze, wagering, food - and it has made your life unmanageable.
Then there's Adam Phillip's understanding of growing up, finally. He's a psychoanalyst. His book is "Missing Out: In Praise of the Unlived Life." He posits that parents figure it's their job to make their children feel special. That's the platform assumed for development.
Out there in the world, there will be growing evidence that the developing human being is not special. When that's embraced as a reality, then the work of growing up is done. After all, even celebrities have to get it that they are not special enough to dodge death.
Grown ups tend to have an easier time of it than those who continue needing to feel special. Also, they are likely to hold onto jobs, stay out of jails, and put together intimate relationships. But most importantly, they aren't the ones manipulated by political leaders, bosses, and romantic cons who prey on their conviction that they are special.
How to make that leap into emotional adulthood? A useful mantra when breathing and and breathing out could be "Not Special. Will Die."