With them being aliens, the whole concept of being a teenager becomes well...alien. Certainly can't be ninjas either because that is an earth thing for earth people on earth. Being alien prevents them from being labeled turtles too...another one of them earth things.
So basically, this movie will be 'Alien Warrior Turtlesque Beings of Youthful Equivalency'
Yup.
--
There is no shame in not knowing; the shame lies in not finding out