I've read that teacher effectiveness rises for the first few years (say 3 to 5) but then plateaus. Some studies confirming this are cited at

http://winginstitute.org/Graphs/Staff/What-effect-does-experience-have-on-teacher-effectiveness/
What effect does experience have on teacher effectiveness?

http://tntp.org/ideas-and-innovations/view/teacher-experience-what-does-the-research-say
Teacher Experience: What Does the Research Say?

If there is no evidence that teachers with 20 years experience are better than those with 5, the two sets of teachers should be paid about the same. Current salary structures negotiated by unions heavily reward seniority.