>looking back, there was no single defining feature of bad teachers I had, but every one of my good teachers were over the age of 40, had previous careers/jobs, and not a single one had a degree (in teaching)..

One of my favorite teachers my drama teacher, probably somewhere on the younger side of 25-35. The year I quit taking drama classes was the year that she was forced to go back to school because she "wasn't qualified" to teach permanently, having not completed a teaching degree.