I've met more than enough 25 year olds with the IQs of larvae, so I don't know what a voluntary age change to 21 will make much difference.
Any move by the industry to 'legitimize' itself in the eyes of the far Right will only be temporary. The moment the FBI find one fake ID of a 19 year old claiming to be 21, it'll be game over and the jack boots will come marching in.
The only way to prevent it is to get the GoP out of office and hope that Hillary likes to wach