It is difficult to convince her parents for this but you do not have any other option also. Both of you should go together and disclose the facts to them. You may have to face anger, humiliation etc. Be prepared for any such negative response.
Difference of age will start bothering you once lust is over and that will increase as years passes. Your insecurity, possession and expectation will also increase with time. She may find that you are lacking enthusiasm after a decade. Your needs and her needs will be totally different at various stages of life that you should keep in mind.