Bran is seven years old.
(And we are in a society where seven is considered an appropriate age to witness an execution.)
(This is also the first mention of the seasonal cycle: “It was the ninth year of summer, and the seventh of Bran’s life.” Is it ever explained what a “year” means in this context? Presumably a certain number of lunar cycles? Yes, I know I could probably look this up at Westeros or Tower of the Hand, but I want to wait and see if it comes from the horse’s mouth in this or a later book.)