Now that TDKR has stealthily communicated why Robin is such an integral addition to the Batman character, I totally think the next movies should have Robin.
I think around 15 years old to 20 is fine as well. If they went on the younger side then only if the kid is already a formidable martial artist (perhaps he does underground street fights to make money (dealing with his built up anger, always decimating the opponent within seconds/minutes)), and only if Bruce trains him to be a ninja, relying on stealth.
Alfred should draw attention to the fact that it's insane. Bruce should seem a little insane by doing it, though we can see that his intentions are good.
Alternatively, if it's an older Bruce Wayne (even Nolanverse), then perhaps there has already been Robins (or Batman successors) before. In the movie perhaps Bruce discovers (when he comes back to Gotham for the first time in 14 years to bury Alfred), that he has a son, conceived when he slept Talia Al Ghul (and born prematurely after 4 or 5 months if in the nolanverse (these premature births have happened before, they just require the baby to be in NICU)), and raised by the League to be the next leader (but recently rescued by Blake).
(sort of like this, but older, and with a dead Talia.)
(This revelation throws a wrench in Bruce's 'retirement' and decides to stay in gotham). His goal is to turn his twisted and damaged son into something positive and productive, and his answer to turn him into a crime-fighter. (Damien could take the name Robin from Blake, to spite his father).