I also wholly fail to understand why an 18 year old cannot enter into a valid NLI without a parent or guardian signature, but that is the official stance of the NLI program:
Question: If I do not live with a parent or legal guardian, is it necessary that a parent or legal guardian sign the National Letter of Intent?
Answer: If you are under the age of 21, your parent or legal guardian must sign the National Letter of Intent in order for it to be considered valid. If you are 21 years of age or older, it is not necessary for your parent or legal guardian to sign the document.