Hey, if they were 15 years old during the NES days, shouldn't they be called the "Mid-Age Mutant Ninja Turtles" by now?
Mirage is also developing this handheld version of TMNT, along with the console versions, and it's just as good as you remember it. The game plays similiarly to the console version, where you need to beat up baddies, collect pizza to regain health, and avoid obstacles. Of course, it's been designed to better fit the GBA in the 2D side-scroller fashion.
Your turtle has a weak and strong attack, as well as a special attack. Weaks can setup strongs in combo fashion, and excecuting a special attack takes away a bit of your energy, exactly like it did in the original NES sequels. There are also some platforming elements included as well, where you'll need to jump across gaps by dashing at the proper time, or doing special jumps to vault over walls (using Donatello's staff, for instance).
There weren't any noticeable problems with the game, other than the fact that it's a bit on the difficult side. This might make it hard for most to just jump right in and start doing the turtle power thing, but then again, Mirage did bring us TMNT in the first place, and those that know that one can say it wasn't easy, either.