The answer is in itch.io's terms and conditions:
If you register an account to sell, distribute, or publish games or other content on itch.io, you agree to be bound to the terms of this Agreement as a platform publisher (“Publisher”). Publishers affirm that they are either more than 18 years of age, or possess legal parental or guardian consent, and are fully able and competent to enter into the terms, conditions, obligations, affirmations, representations and warranties set forth in this Agreement.
I'm not sure what kind of proof is required. Contact support if nobody chimes in. Other people have done it, so somebody must know.