It's quite a challenge to give level 1 characters something to do that doesn't kill them. But its still fun, and once the story is crafted its really easy to run.
Starting at second level, the things you can do with the characters opens up dramaticly, and stays wide open until the mid-levels. In 1st edition, that used to be between 6th and 8th. After 8th, the PC's would quickly gain world shattering power, and by 12th were more than capable of dealing with just about any monster in the book. At that point, it became just as hard to challenge the players without killing them as it was at 1st, but the encounters were much more difficult to run. Organization and work load just gets to be a problem at high levels.
For third, since the monsters are more powerful and the power progression smoother in all the PC classes, I think you can stretch the mid-levels out to 12th or so, but beyond that I think that things are going to get really hard and the balance between the classes would probably start falling apart.