In some limited circumstances I do agree that some juveniles should be charged as adults. This should, however, only happen in the cases of the worst crimes and the oldest juveniles.
The whole idea of having a separate juvenile system is that juveniles are less responsible than adults. I think that this, on the whole, is true. Juveniles are more likely to do things on the spur of the moment that they would not typically do. Therefore, it seems to make sense that they should have a separate system of justice.
However, some crimes are too heinous to take lightly. Murder, of course, is one. Certain types of sexual assaults might qualify as well. Juveniles charged with these crimes should often be charged as adults if they are in their late teens. In other words, a 14 year-old should not likely be charged as an adult regardless, but a 17 year-old who commits a truly heinous crime should be.