Do university teachers, like good wine, improve with age? The purpose of this methodological/substantive study is to apply a multiple-level growth modeling approach to the long-term stability of students' evaluations of teaching effectiveness (SETs). For a diverse cohort of 195 teachers who were evaluated continuously over 13 years (6,024 classes, an average of 30.9 classes per teacher), there was little evidence that teachers became either more or less effective with added experience. This stability of SETs generalized reasonably well over undergraduate- and graduate-level courses, early career teachers, and teachers who differed substantially in terms of their teaching effectiveness overall. Whereas there were substantial individual differences between teachers in terms of their teaching effectiveness, these individual differences were also highly stable over time. Although highly supportive of the use of SETs for many purposes, the results provide a serious challenge for existing programs that assume that SET feedback alone is sufficient to improve teaching effectiveness.