Nope, only after the season during which you turn 18. Obviously, it's because they want to have people qualified for the same category all season. Of course, that means it would be unfair to let a 17/18 category competitor run unrestricted just because they turned 18.

I do think the roll-out rules for juniors in senior races is crap, but it's a non-issue in most races. It only matters in road races with long fast descents. And I don't wanna hear "but a good racer can spin 200 rpm" crap. Nobody likes to spin 200 rpm for a minutes on end, and if they did it would surely wear them out quickly.