Let me explain u the legal age of filing of petetion for I-129F. This is a guranted 100% correct reply based on my experience i have also faced same issue for my visa as well. There is no legal age for filing of visa petetion remember it. If some one want to marry in US he or she must be legal age as per the age requirement of the state. some states allow 16 years marriage with parental consent and some states dont allow. 18 year is a free age for marriage in usa. However incase of visa processing every visa applicant has to file affidavit of support I-864 or I-134 under section 231-A of american citizenship act therefore at the time of interview of applicant at the embassy the petetioner should be 18 years of age other wise counsellor officer will refuse visa uder section 214 of american citizenship act. 18 years age is mandatory for a sponsor to fill up I-864 thats why the visa is declined. If you need any other information feel free to ask.
There is a requirement that a person be 18 to sign the I-864. The OP doesn't meet that requirement.
And the I-864 would be required from the petitioner once the fiancee entered, they married, and applied for adjustment of status.
