I wouldn't worry about the age difference. My wife is 21 years younger than me. Just make sure you have good supporting documents to back up your case. We had photos together from 3 different trips I took to meet her and her family. I also included about 2" of Facebook and Skype chat logs. If he has a receipt or apprasial for the engagement ring, include that. I would also include any wire transfers he's made to your bank account.
SWTI37 got a reaction from winternev in K1 visa filing. We met once for 3 weeks, filed the application just a few weeks ago.