Eventually one of them will die in the attempt to be the youngest. There will be a lot of hand wringing then. To be honest, it's not the maturity of the child, I would worry about. But the maturity of the parents. You have to wonder if the child turned up and said 'I really, really want to sail around the world solo'. Or was the idea planted by over ambitious parents. If it was the child's idea, a simple NO would suffice.
It reminds of those stunts where a child flies long distances with an Instructor alongside of course because flying is more regulated than sailing. Remember the one that crashed? That put a stop to it for a while. One or more of those kids will die inevitably and gain the grim record of being the youngest unassisted solo sailor to die pointlessly for the benefit of their parents' egos.