Is there an age limit for fiance visa?

There are no age requirements associated with the fiance visa. With that said, however you have to be careful that there when the foreign national arrives in the US, he/she will be legally qualified to marry. In other words, if the fiance who is brought to the US is under 18, there may be issues of state law which prevent that individual's marriage without first obtaining parental consent. Qualifications to marry are an issue of state law so the answer would necessarily depend on where the marriage will take place. On the question of an age gap requirement, there is no such requirement. I would advise that speaking to an immigration attorney may be a really good idea in your situation. You have not provided enough details to permit someone to figure out what the concern may be in your case and I think that it would be a really good idea to sit down and speak with an attorney who can instruct you on possible issue that you may encounter in the process of applying for a fiance visa. The more information that you obtain, the better off you will be in ensuring that pursuing a fiance visa is the right step to be taken.