Personally I think Dick Grayson is the only Robin that actually works in a Nolanesque semi-realistic Batworld.
I can suspend my disbelief and suppose that Bruce Wayne spent a few years in a ninja camp and can now beat up anyone. I can't suspend my disbelief and suppose that a teenager could have anywhere near enough ability and training to achieve survivability.
UNLESS that teen had been trained as circus performer since infancy. Now we are talking about someone who at the age of 16 has abilities that the average person isn't equipped to deal with. That doesn't make him the world's greatest fighter, but it makes him unique enough to have an actual skillset that has a purpose.
In terms of plot, don't use the name Robin and don't use a costume. Have Grayson and Batman working in parallel looking into the same matter (obviously something about who ever decided to commit mass murder at the circus..) Bring them together for the final act. Make the story play out so that Grayson is going to be "Robin" with or without Batman, which removes the ethical question. I also don't see anything wrong with 16-18 as an age range, Bruce is at least pushing 30 by this point.