Here's my idea:
Which Robin: Have a generic robin, someone who could be any robin, and just have him called "Robin".
Introduction: While answering a routine distress call, Batman finds a crying boy kneeled over two lifeless bodies-his parents. Instantly flashbacking to his own personal tragedy and seeing a resemblence between the two images, he drops the kid off with the police, picks him up the next morning as Bruce Wayne, furthering his image as a philanthropist, and takes him home as a surrogate son.
Becoming a hero: During the course of TDK, Robin accidently finds out that Bruce Wayne is Batman, although Bruce himself doesn't know this. By the end of the film, Robin is considering telling Bruce what he knows.
Towards the end of third film, there is a Sherlock Holmes scene (which there must be to ensure that the franchise continues post-Nolan) in which everyone is led to believe that Batman has been killed. When Robin and Alfred arrive at the crime scene-which happens to be the docks-they find the top half of the batsuit floating in the water. Robin picks it up, stares at it meaningly for a few moments, then tells Alfred that he has been training himself for the past year and a half and plans to follow in Batman's footsteps. Alfred tells Robin that if he does, he will not stop him.
Robin film: Have a Robin spin-off film showing his origins in the circus and his first big battle as Batman's successor.
Age: I don't see why having him in his mid teens is lacking credibility.
Cast: Logan Lerman immediately comes to mind for this job. I mean, his last screen character (Jim Carrey's son in The Number 23) was named Robin Sparrow, for goodness sakes. If he can't do it, Shia LeBeouf, who has come a long way from "Even Stevens", would also be fine, maybe even better.
