I received my first dan at 7. It was in Moo Duk Kwan, Olympic style Tae Kwon Do. It is funny to think that the people who end up most usually evolving the art, who become geniuses at it, are the lucky few who begin at the youngest of ages, and get to experience it throughout their entire life. They get to have the 60 or 70 years we all hope to have, and more. A dan, or acknowledgment of standing within the art, should be awarded based upon ability and insight into the art, and if a 7 year old, a 4 year old, or a 40 year old, why should how long they have existed determine the actual depth of their knowledge? It seems unrelated to martial arts, to me, because our ultimate goal in every technique is to both unify ourself with the art, internalizing it, while at the same time removing ourself from it as much as possible, until our techniques are executed perfectly, and were as if meditating with a still mind. That's when movement flows, and mastery begins.
I can't say I deserved my ranking at 7... but I do think there are some who certainly deserve it, and might be younger than that age.