I want to monetize my videos, but I was disapproved for being under 18
The minimum age requirement to participate in AdSense is 18 years. If you have submitted an AdSense application through YouTube and were disapproved by our system for being an underage publisher, your YouTube account will still be linked to this disapproved AdSense application. You should follow the instructions in the disapproval e-mail you received to verify your age (if you are over 18) and resubmit the AdSense application.
In case you are under 18 years old, the only way to start monetizing your videos is to link the YouTube account to an approved AdSense account (of a parent or guardian who is over 18).