Flieger,
I agree females very comfortable in dresses, but I would add one thing to that comment.
Girls, make sure you break in the shoes. Wearing those pretty pumps that make your feet hurt after half of an hour, will make it difficult for you to concentrate on the questions being and asked while you can feel your feet pulsing in pain.
Also if they are new remember to scratch the soles up because you don't want to walk on carpet and slip because they were brandy new.
For guys,
You may want to think of a 3rd option. Khakis, dark sport coat and a brand new mock turtle neck. This will give them the best of all worlds. There will be no tie to feel constraining around their neck, the sport coat if classic (think navy or black) will never go out of style and will dress up the look of khakis.
It will also give them an age appropriate outfit that they will feel comfortable in while giving them a more respectful image.
Same thing about the shoes for guys as it is for girls, maybe even more so. Guys, at least my DS's wear rainbows, boat decks or sneakers. They are not accustomed to dress shoes. So if you are going for that suit, remember they will wear dress shoes, and they are confining compared to other shoes. Have them break the shoes in prior to the interview.
I also agree about the personal image look. I remember sitting at one for our DS and it drove me crazy because I wanted to pull another kid into the bathroom and tell him scrub under your fingernails. I remember thinking if he offered me his hand to shake I would feel uncomfortable or at the very least look around for a bottle of Purell. Now maybe he had an excuse (worked pt as a mechanic, or landscaper), but overall I could not stop staring at his hands.
Girls same for you, no chipped nail polish or insane colors with long tips that make it impossible for you to dial a phone without a pencil. People look at hands, it is one of the 1st things they see as you extend your hand to shake theirs. Some people like me get caught on it, just like Flieger got caught on the rats nest of the hair.