For a while, the film was absolutely 100% going to be called just Ninja Turtles. This suggested that the lead characters would be neither mutants nor teens.
It's been hammered home by the marketing that they are mutants, turning around original plans to have them be aliens. Paramount really want to turnaround bad word of mouth on that front.
But it hasn't been clear, due to a general lack of understanding of mutant turtle physiology, if the characters we're looking at are teenaged or not.
Not until now, when images of teen versions of the characters – in toy form, of course – have arrived. Here's a shot from CBM.
So, it seems like they will be appearing as teens, but don't end up that way for the 'meat' of the story. Or maybe they're young teens in the mid-form, older teens in the grown form? That would make sense of the more recent decision to retitle the film with "Teenage" included again.
Anyway. I don't care, personally, I just know some other people will. As far as I can tell, though, this isn't going to impact on the quality of the film, and so I don't have any investment in it.