What exactly do you mean? If you are referring to age difference between US citizen and fiance, a large age different is not by itself disqualifying but it may mean you would want to provide extra documentation to prove the nature of your relationship.If you are referring to the US citizen, the U.S. petitioner must be at least 18 years old, and be able to prove he or she is a United States citizen.If one of you is under the age of 18, you are likely to be considered underage in the United States. Your legal ability to marry will depend on the laws of the state where you plan to get married. Each of the 50 U.S. states sets its own rules, and you will need to research them. For example, you may find that in one state you must be 18 years of age to marry, while in another you can marry younger if you can show the consent of your parents. If one of the applicants is under 18, you may want to consider waiting. While you wait, gather documentation proving your relationship such as photos of the two of you and with friends/family and travel itineraries for when you visit each other.Also, for your general information on K1 visas, I am going to paste an answer I previously gave because it is a very detailed outline of the process.Having served as an Executive Assistant for a former US Department of State Foreign Service Officer and Consular Officer, as well as worked in an immigration law firm and for an immigration help center, I believe I can help. This does not serve as legal advice as I am not an attorney. While form instructions are available on the USCIS website for each form, immigration law firms also often provide free 30 minute consultations.The fiancé visa is has an unusual visa application process as it is technically a temporary non-immigrant visa, lasting only 90 days. During this time the fiancé and U.S. citizen sponsor are to be married and she will adjust status from the US. You can also check processing times here USCIS Processing Time Information. At the California Processing Center, they are currently processing applications filed May 1, 2017. So If you submit your application December 1st, you can expect about a 7 month wait before you hear back. I have answered similar questions, so some of this will be copy and past but still very relevant to your inquiry.You are required to complete Form I-129F, Petition for an Alien Fiance. A money order of $535, made payable to the “U.S. Department of Homeland Security” is required at the time of submission. It is crucial that you make the money order payable to “U.S. Department of Homeland Security” rather than USDHS or any other variation. You can staple the check to the application at the time of submission. See this USCIS website to find where to submit the application: Direct Filing Addresses for Form I-129F, Petition for Alien Fiancé(e).Once you receive the USCIS submission receipt, you can complete form DS-160 online with the receipt number. After that is submitted, you will acquire documentation to prove the nature of your relationship as well as Form I-134, Affidavit of Support. At this point you will need to acquire documentation specific to the Embassy/Consulate your fiance will be interviewing. Here are some basic ideas regarding what to bring and how to bring it:· Organize your documentation and label it so that it is easy on the Consular Officer’s eyes. Remember, he/she handles countless interviews a day. Make this one easy on the consular office. If you want to would recommend what many law offices do: Create a table of contents; separate each section with a different color of paper which has the section labeled. This makes it easy to thumb through; bind the papers neatly together with a clip or fastener.· Form I-134 as well as proof of your income as a Legal Permanent Resident such as taxes from the last 3 years. Search USCIS I-864P, HHS Poverty Guidelines for Affidavit of Support, to see if you meet the financial requirements to sponsor your fiance. If you do not, you will need a co-sponsor.· Copies of marriage and divorce certificates for all previous marriages involving yourself and your spouse.· Copies of all of your forms and related documentation (specifically: I-129F Receipt, Form I-134 Affidavit of Support, DS-160 Confirmation Page, other submission receipts, appointment notice, your fiance’s non-expired medical and certified police record)· Documentation proving the nature of your relationship. This can include but is not limited to: photos with fiancé(e) on various trips, engagement celebration, proof of engagement ring purchase, announcement to friends/family, affidavits from friends/family/employer, previous travel itineraries, phone records demonstrating regular communication, letters to and from fiancé(e) (regular and special such as birthdays or anniversaries).· Check the embassy website for your country to see if additional documentation is required for your location.Finally, prepare for the interview itself—This is crucial, especially in the current political climate. Although exact numbers are hard to come by, I have read that anywhere from a quarter to one-third of visa applications are denied at the interview stage. This varies widely depending on the consulate, consular officer and country from which you are applying. Unfortunately, many people are denied visas due to unpreparedness or nervousness at the time of the interview. It is understandable: a lot is riding on one quick interview. I have found a service that seeks to increase your chances of a successful interview by connecting applicants with experienced former foreign service officers for one-one-one practice interviews. Check out the website here: www.visapreponline.com and let me know what you think. I wish I had known about this service several years ago!Please keep us updated and good luck!