The Learning Policy Institute’s research review paper:

Does Teaching Experience Improve Teacher Effectiveness? confirms and adds to commonly held assumptions about the impact that longer serving teachers can have.  The work is based on a review of 30 studies published within the last 15 years. Very substantial.

Here are some of the headline points:

• Teaching experience is positively associated with student achievement gains
• Gains in teacher effectiveness associated with experience are most steep in teachers’ initial years, but continue to be significant as teachers reach the second, and often third, decades of their careers.
• As teachers gain experience, their students not only learn more, as measured by standardised tests, they are also more likely to do better on other measures of success, such as school attendance.
• Teachers’ effectiveness increases at a greater rate when they teach in a supportive and collegial working environment, and when they accumulate experience in the same grade level, subject, or
  district.

Two of their recommendations:

• Increase stability in teacher job assignments [allocation].
  Teachers who have repeated experience teaching the same grade level or subject area  improve more rapidly than those who are often moved around year groups or subject areas.
• Create conditions for strong collegial relationships among school staff and a positive and professional working environment.