That's a bit harsh. If you want a nice, well-appointed, well-staffed martial arts school to train at and want it to still be there 5 years from now, your instructor will almost surely have to teach kids. I've seen too many adults-only schools that moved from a studio to a garage to non-existent as the studio's bills mounted and the relatively small number of dedicated adults (plus the slightly larger but fluctuating number of interested adults who came-and-went as jobs changed, marriages occurred, etc.) couldn't support it. It may be to your benefit as serious adult to have kids at the school too. Plus, your instructor is an adult with a family and deserves a good living just like anyone else. Teaching kids may mean he or she has enough staff to qualify to offer health benefits for his or her employees.
As an imperfect analogy: If you like pro baseball, you need to understand that it stands on the backs of minor league, college, high school, and ultimately Little League ball. The kids you find boring to watch and undisciplined now will be having their HRs reviewed by instant replay some day.
I too prefer an adults-only school. But that's a tough business model. In a large city or a college-supported club you can have some of those. In a smaller town it's a lot harder.
Now, ranking issues are another matter and I'm of the "at least 16 years old for black belt" school of thought. Kids need frequent reinforcement, though--like daily gold stars in kindergarten.