All the parts are played older in the stage version because the premise is that they are reenacting the story for us after the fact. Sweeney rises from the grave and they all tell the tale. This was portrayed more clearly in the revival with Toby "telling" the tale from the asylum he ended up in at the end of the tale. But the original also hints at this conceit.
Toby was, in fact, a boy when the story they are retelling/reenacting on stage took place. Since the movie depicts the story in real time, as it happens, Toby should be played by a boy. And Sweeney and Lovett also appear younger than we are used to.
Those clips give me nothing but hope. I am so excited for this. I could not have imagined that musical films would make a comeback, and Sweeney would have been the last one I thought would be made. I feel like I'm sharing a special treasured possession with the rest of the world.
Art has a double face, of expression and illusion.