Rival has no minimum age required to signup and participate. 

However, players under the age of 16 will need their parents or legal guardian's permission and acknowledgment. Please see the "Terms of Service and Privacy Policy" section for all relevant policies.

There may be specific events, tournaments, or Rival areas that do have a minimum age required to participate.  If you are not of appropriate age, the Rival system will provide that message when and if you attempt to participate before you have met the age requirement.