It's an interesting question. Look at it a different way - if a volunteer was released from service due to being under age eighteen, would he receive a permanent exemption from service? I don't believe that's how things worked.

Do us a favor and check Confederate law on that topic. If I'm not mistaken, the next age limit for conscription was forty (October 1862), but I've not seen the law. I'd really like to know what you can tell us about this.