Appropriate, polished dress, and good personal hygiene are elements of the job interview that are easily conquered, but those only touch on the surface of how you should present yourself when meeting a potential employer. When you're obviously younger than an interviewer, there can be a tendency to take on a subordinate role in the conversation.
Do your best to present yourself as an equal, no matter the position of your interviewer. Of course there's an understanding that you would potentially be working under this person and your questions will show that you regard the person as your senior. But attitude is more than the basic understanding of where you stand. It's about showing the interviewer you're confident with what you have to offer and can hold your own when speaking to anyone.