Does THAT really need to come out as an actual plot device tho? Readers know, on a subscouncious level, that characters dont really age as we do, unless the writer makes it so. It all sounds like trying to explain why the Sun always rises up.
Not to mention it all sounds a bit Phantom-ish, wich only kills it further. Batman is no Phantom.
That's not his intention either. Morrison isn't saying Bruce is immortal. He's saying that the legend of Batman is timeless. The character will never age. He will never evolve like people evolve. The character will never become like he is in "The Dark Knight Returns", "Kingdom Come" and "Batman Beyond". He will always be the same character that he was in 1939. The same age. What's happening with Bruce's return has to do with how he adapts to different enviroments across different ages. Once he returns to the 21st century, we'll see how he deals with a return to normalcy.