Sure, you are what you are __now__, but as a teenager that is going through radical changes. That's why you can't even get a tattoo in the US until you turn 18. In some states, not even with parental permission. Because it's irreversible. The whole trouble with reassignment surgery for minors is precisely due to the fact that who they are today is not who they will be as adults. Of course people should be treated as they want to be treated __now__, but society recognizes that adolescents are not capable of seeing far enough ahead to adulthood to be entrusted with making certain kinds of permanent, life-altering decisions.

There's no debate I'm aware of about lowering the age for getting a tattoo, or drinking, or acting in porn, or other things which are fine for adults, but which we don't permit 12 year olds to do.