I am myself, and I can define myself however I choose. If I chose to define myself as an adult then the limits of my mind will allow such.
In order to become a member of society every human must at some time accept that they will adhere to strict social clauses and rules that may define oneself. If one chooses to redefine oneself as an adult then they must restructure their soul and comprehension of the identity which contains them. The restructuring of one's identity is something that may change the 'I' in one's sentence, and you may lose identity altogether. In some cases, one may have never achieved an identity to begin with.

I am me.
You are you.
I'll leave it at that.