Look at the title for the body because my question is short.
Every licensed amateur, regardless of membership in ARRL or any other local or national organization is eligible to apply for membership in ARES.
So, if you have an amateur FCC license, you may apply for ARES membership.
From Amateur Radio Service:
The amateur and amateur-satellite services are for qualified persons of any age who are interested in radio technique solely with a personal aim and without pecuniary interest.
There is no age requirement for getting an amateur FCC license.
Keep in mind that the ARES page also says:
Please inquire at the local level for specific information.
So, your local ARES organization may have an age requirement for participation, but you will have to ask.
Many activities in ARES would probably require a legal age adult. However, few of those activities would exclude someone younger if accompanied by their parent or guardian, and many activities are easier with two people.
A perfect example of this would be amateur radio communication for a hurricane storm shelter, where ideally one person sits with the radio and the other person acts as a runner to relay communications within the shelter. If both people have sufficient training and licensing, the roles could even trade off.