Running a C-Corp is no easy matter, I would suggest first you look very hard at why you want to do a C-Corp vs an LLC with an S-Corp election. There is a lot of documentation/formalities that if not followed allow somebody to "pierce the veil" and bind the principles legally. You also miss out on tax savings opportunities with recent legislation and subject yourself to double taxation, which is beyond the scope of this question. Think carefully.
To answer your question, the person who signs the documents is the founder/former of the corporation. You can file paperwork to remove that person from the corporation, however in your case I would not do that, and here's why...
One of your principle shareholders will be a minor. A minor cannot be held to the same contract standards as majority adults. This means that many organizations that would otherwise gladly do business with you (like a bank account) will immediately turn away. You need an adult to bind the company and all the principles should be legal adults. You cannot allow your minor business partner to be part of any contract or a party to a signatory on a contract.
Yes, a minor can hold "shares" of a corporation, but they are severely limited in that they carry no voting rights. They do give the holder dividends, but there are tax implications there too that the corporation needs to be careful of.
You will need to disclose that your business partner is a minor in your dealings, not doing so can open you up to all kinds of trouble. That alone is enough to make many organizations walk away. I would suggest that you keep all your principles as majority adults and draft documents that transfer the shares to the minor upon the age of majority.