Once you reach independent adulthood, the age difference doesn't have to matter. Although the less time he/she has spent as an independent adult, the less prepared the younger person is to meet an older person on an equal footing.
If you are within a few years of each other's age, you are usually better able to understand each other because you grew up in the same larger culture - generations do have overall identities - and when one of you refers to a tv show or video game or a popular band or something, the other one will usually know what you mean. And you are more likely to have similar ideas about your roles.
There are other ways to start building a common identity, but having similar childhood experiences due to similar age is an easy starting point, and can be very comforting.
