All members of your Accelerator team need to be 18 or older as per legal requirement. Other members of your development team who won’t be in the Accelerator program do not need to be above 18. You can still apply to the Accelerator program with the members of your team that are 18 or older and then work with other team members who are younger than 18 during the program.
For reference: my Accelerator team was 5 members all above 18 but our total development team is worth 10-12 people, two or more of which are not 18. Those two (or more) did not join us during Accelerator however they continued to work on the project alongside us.