I couldn't find any particular information on Itch.io documents(FAQs, Terms of Service). As stated in Terms of Service, you must be 18 to publish a game, does it apply for jams?
Considering, that participating in a game jam means publishing a project, the same rules apply by implication.
Note that that’s not the full quote (emphasis mine):
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