While I have never seen it in the rules, a common minimum age for joining forums is 13. On MySpace, it is 14. Generally, people that are 13 and up should have no problem, especially if supervised by a parent. Even people that are younger can post if supervised by parents, if their parents do not object to the minimal amounts of swearing and adult themes one might find on this forum.
However, Simon is the one to make the final decision. If he says it's 13, then so be it. If he says it's 18, then it's 18. And so on. If he doesn't specify, then it's really up to the parent to decide if the child is OK to be here (which is pretty much the smart thing anyway). In any event, no one can be responsible for anyone that willfully misrepresents themselves as being older than they actually are just to be permitted to join the forum. I do not recommend that, regardless (and, ultimately the decision is better left up to the parents than the forum owner anyway).