Yes, the rules only allow for athletes aged 16 or 17 in the year of competition to enter. Only athletes born in 1996 or 1997 may enter next year's youths championships.
Interestingly, athletes aged 16-19 in the year of competition may enter the world junior championships.
Amusingly, we had an athlete DQed after winning the national junior XC about 2 years' ago for being underage. I understand the concept of protecting young athletes from the stresses of "overage" competition, but this athlete was by far the most comfortable in the field, and won by nearly a minute.