However, I definitely agree that the live-action films were great. I remember loving the first one, and liking the second (the third was a disappointment IIRC).
The second was my least favorite; it was too kiddified and goofy, too toned down. The third film was somewhat smarter and between the first two in tone. Its main problem, though, was that the producers wouldn't shell out (ouch, pardon the pun) for Henson's Creature Shop and the animatronics were greatly inferior. The Turtles' mouth and eye movements were stiff, jerky, and obviously mechanical. (Although it wasn't as bad as the turtle heads in the live-action
Next Mutation TV series, where you could clearly see the masks' neck seams and eye holes.)
But the turtle suits and creature f/x were outstanding. Everyone thinks that CGI is the way to go, but I disagree.
I think the idea here is to use animatronic suits but replace the faces with digital animation. I think that's probably the best way to go in terms of realism. But realism isn't always the goal of art. The 2007 film wasn't trying to be realistic; it created a stylized, graphically rich look that I found quite effective (aside from perpetuating an unrealistic and unhealthy body image for women). There's nothing wrong with a comic-book movie embracing its roots in cartooning. I think animation is a good medium for TMNT.
The '07 film was also a good movie for fans of the '03 animated series. Its approach to the characters and the story had a lot of similarities (probably because both were emulating the original comics), and the Turtles' voice actors sounded a lot like the ones from the TV series (though they were all different performers).
As for the question about whether the Turtles can validly be called teenagers anymore, maybe that's part of why the '07 film was just called
TMNT instead of spelling out the full name.