Wycoller is correct to point out that Grasmere, which is an FRA race, states that the junior ages are age on the day of the race. The race organiser always stresses this.
However, the race organiser is not breaking the FRA distance rules because the junior races at Grasmere are so short that the youngest runner in each grouping will not exceed the distance allowed even using age on the day of the race - see my earlier post.