When you're young, you wanna look old. When you're old, you'll do anything to look younger. The latter is especially true with the advent of high def.
But this is for those who have just graduated or still in their early twenties. And I speak from experience, because I looked so young I was stuck in radio until I was 28. I routinely got carded well into my thirties. (Now if someone offers a senior discount I get seriously ticked off.)
It's hard for people who look very young to have credibility, so early in your career you need to add a little age wherever you can.
Men: Wearing polo shirts or shirts with open collars only makes you look like you're still in college. Neckties are a must, and dark suits will make you look older than light colored ones. Old school stuff like cuff links and pocket squares can also add a bit of age.
Women: Stick with business attire; suits, jackets, professional outfits. Forget things like sleeveless blouses or dresses or stuff that's too trendy. Hemlines should hit the knee. And nothing says old school more than a simple strand of pearls. (They don't even have to be real.)
When you get older, looking young will be a nice problem to have. But for now, age and credibility go hand in hand, and you need all the age you can get.