It's really not all that unusual. History is littered with amazing minds that accomplished tremendous things well before adulthood. The modern fiction that humans are useless until adulthood is one we have to construct by force, through active neglect of education, denigration of intellectual pursuits ('Go outside! Get a life! Do anything else but read or sit and think!'), and adamant insistence that it is normal to be ignorant and incapable until adulthood. Pascal recognized his triangle as a youth, Gauss was similarly known for early feats, etc. Those are just accidents of history, mostly, that they are remembered. The actual number of fertile and productive minds is doubtlessly orders of magnitude higher. It's only through diligent refusal to believe, or to tolerate the notion that maybe we should have expected a bit more of ourselves in our own youth, that we can keep up seeing intellectual achievements of the young as an unusual occurrence.
Unfortunately it was cancer from the radiation of atomic bomb tests that killed John von Neumann far too early, not his experiments with cellular automata.