To answer to the question in the original post, I guess it's around 20 and 30 years that you are defined at 90% (lifestyle, opinions, beliefs, etc), and you can call that "yourself". It all depends on how early you did realize what is surrounding you, if you are aware of that world at 15 in its , you'll be accepting the world like it is at 20, and find "yourself" around 25 years old. Still, people who have one or several major events in their life can be "broken", and change almost entirely, become someone else, that they can call themselves afterward, without being proud necessarily of it. Traumatized soldiers for example need to accept the world again, and build back themselves from nothing.
My opinions thought.