You are a junior until your racing age is 18. All juniors can do senior races in their respective category, but they must roll out. I don't agree with this recent rule change and if they don't win or place, I don't see why a junior would even go to roll out after a senior race. Juniors who have a racing age of 17 or 18 can do Senior nationals, which is why Taylor Phinney was allowed to race and win the pursuit at senior nationals last year and the same reason Rahsaan Bahati and Keith Norris both were allowed to win the elite criterium nationals several years ago, both as juniors.