Focus on mutual interests. Any relationship will benefit from finding out things that the two of you have in common. If you both enjoy the same cuisine, travel or film genre, spend time doing these things together. These common activities will keep both of you from dwelling on the age difference.
Try new things. While establishing common ground is important, it's also beneficial to try things that your significant other is interested in that you may not be familiar with. Let them take you salsa dancing or out for sushi, even if you're not sure you'll enjoy it. Meeting each other halfway and exploring each other's world is part of what makes new romances fun, even the "May-December" kind.
Discuss your uncertainties. Whether you are the older or younger half of the relationship, you are bound to have issues or doubts as to whether or not the two of you can actually make things work. If you don't address these issues and come to a mutual understanding about the problems that both of you are having, the age gap between you will be exacerbated and become a problem.
Make them part of your circle of friends. Both of you will feel more comfortable in the relationship if your friends and the people whose opinions you value meet your significant other and come to understand why you like or love them. Getting to know your loved one's friends and acquaintances will also allow each of you to become more involved in each other's life which will bring you closer together.
Ignore the naysayers. No matter how well the two of you get along, there are bound to be people who will judge you because of the age gap between you and say negative things. They might say that one of you is robbing the cradle or that the other is looking for a "Mrs. Robinson," but it doesn't matter what anyone says if the two of you are secure in your feelings for one another. Agree that you will both try to remain unaffected by these misunderstanding people and focus on the fact that you care for each other and are happy in the relationship.