The local paper ran a rather bland article yesterday about the number of new teachers in local schools ("School debate value of novice teachers").
According to the people they interview, it essentially boils down to this: newer/younger teachers are more enthusiastic and sometimes relate to the kids better, while older/more experienced teachers have better classroom management, more expertise, and can serve as mentors. Sounds about right to me.
I think you can make a compelling argument that too many of either one is a bad thing. But where is the line for "too many?" This particular article highlights Rutherford County (in the suburbs of Nashville) as having the highest number of novice teachers (due to rapid growth) in the area. 37% of the teachers in the district have five or fewer years of experience.
That's fairly high, but about half what it was in the school where I taught (which was not atypical for middle schools in the Bronx).
I would think that having around one-third of the staff in their first five years (assuming that there's an even distribution within that experience demographic -- e.g. not all of that third are in their first year of teaching) is probably about as high as you would want in an ideal school.
A school where less than a third of the teachers have five years of experience or more, however, should be raising somebody's eyebrows. Not that it's not possible to have a successful school in such circumstances, but it certainly merits further investigation if you're an administrator.