* Daniel Radcliffe played the older Harry Potter, for the majority of the film.
* Daniel Radcliffe also played the younger Harry Potter, in scenes where they needed to show a younger version of Harry, for example, when he was a baby.
* James and Oliver Phelps also played baby Harry in one scene, where baby Harry is crying in his crib.
It was a bit confusing, even for the filmmakers! It was difficult to get one baby to act in all the scenes, so they had to use multiple babies to play the character.