Unless your birthday is December 31, your racing age will always be the same or higher than your own age... i.e. you won't be ahead of your racing age and still have to do rollout even though you are already 19. The earlier your birthday falls in the year, the longer you will be a senior while you are 18. I think that's why you have to be 19, so that no 17 year olds get moved up early...
Probably right, and maybe a liability reason as well.
My birthday is in Jan so by the time the season gets underway, my actual age is my racing age the whole year. That was seen as an advantage as a junior, but a disadvantage as a master.