They showed the turtles growing up as a flashback in the first one, so if Shredder wants useful muscle now it'll pretty much have to be adult human bruisers turned into mutants.
Unless they do indeed make some dumb "fast acting" mutagen that somehow makes them full size and brained. As full brained as those two can be, anyway. Which since that's a dumb idea, I won't be shocked if that's the case.
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