Depends how old Teddy is meant to be in the picture. Crowe is getting on a bit to play a young, young man but make up would allow playing somewhere in the 30's to somewhere much older (see A Beautiful Mind)
DiCaprio (after whole bunch of taters, which is something he did for Gangs of New York) might make a better choice for a younger Teddy. Also, it wasn't The Aviator that made me realise he had talent. It was What's Eating Gilbert Grape which I saw after seeing Romeo and Juliet, although he made it earlier, that made me realise that he was a little more than just the latest movie pretty boy.