Is it right that dance teachers are expected to teach all age groups?

One thought which I've been pondering is that a 'qualified' dance teacher in the private sector is expected to teach competently and effectively to all the age groups.In the academic sector there can be very few teachers qualified to teach their specialist subjects with equal skill and expertise to early years, primary, secondary,higher education, undergraduate and Third Age students.And yet this is the common expectation of most dance teachers in the private dance sector. In practice many dance teachers do have a preference with regard to the age group or standard of classes that they teach. In most cases this can be put down to where they feel the most comfortable, confident and effective as teachers.It might be teaching a particular age range or students who have gained a certain mastery of technique.
There is no option as far as I am aware in the private dance sector for a teacher in training to specialise in a specific category of dance training eg Early Years or Secondary. Would this be a welcome option and if it were possible how would it impact on dance teacher training, employment opportunities both in the private and academic sectors and consumer choice.