Can a minor (under 18) register a Commission Junction account if they

Legally you have to be over 18 to be able to sign up to these services although varies from country to country.

If you really want to sign up and use this service I would suggest you get one of your parents to sign up and then use their account for what you want or you may be able to get them to put your parents as the account holder and you as the payment person and get access to the account but varies from service to service.

If you are still unsure then I would suggest you contact commissionjunction.com about your issues/concerns and see what they say.

When you signup they ask if your 18. You could either signup under your parents name or else contact cj and ask them what can be done:
https://www.cj.com/about/contact.html