Life isn't defined by the number of years on a person's belt. I'm not sacrificing a man for a child just because the man has had longer to live. That's asinine. Nobody should move on the assumption that just because someone hasn't experienced life that their life is automatically above that of someone who has already lived. Because that's false; there is no good answer to this question unless you move on the assumption that, A, the old man will fulfill his commitment, or B, the kid turns out to be an extremely significant figure in the future. Neither has a guaranteed quality of life if you were to choose to save them. Either one could die from unfortunate circumstance the week after, for no good reason.
No reason has been given that would put either one of their lives on a different moral scale from the other. Youth is not a moral virtue.