Professional teachers specialize on different age groups for varieties of reasons; one is the mental state of learners, another their capabilities and then there are a few others. All of those are addressed in schooling of teachers. However teachers who carry out their duties with varieties of ages and/or goals of students ought to be admired, their employers/institutions should be looked at closely.

A really professional ESL teacher should not only be able to teach all levels of ESL
students - from younger to older, at Middle and High school and in (non-profit) adult education h/she should also be able to teach in a variety of cultural settings.