It is not so much that they do not have the right to make a contract, it is just that they are legally allowed to break the contract, because of a lack of informed consent, if they decide they made a mistake. This does, however, make it difficult for them to make a contract, as few out there are willing to risk a great loss, should the minor disafirm the contract. A 16 year old might go buy a car, total it that day, and bring back the remains and disafirm the contract, leaving the dealer without his product or the money he expected to receive from it. This is why a guardian is usually expected to sign as well, so that the guardian can be held accountable. But don't think that minors aren't allowed to contract. They can, they just aren't expected to be held to it.

Be wary when suggesting ancient laws. Though they might be reasonable, we live in a society where the further back in history you go, the less accepted the people are. Roman law would mean little to someone of today. Roman law would seem quite worthless and "backwards." You will be presented with evidence of great advancement and proven Roman misconceptions. Remember that children have been "scientifically proven" to be inferior creatures. Don't expect any legitimate source to back up the claims you will be met with, but the claims will be made.