Anyone who's is able to board the aircraft without needing some kind of assistance shouldn't be allowed to pre-board. The only exception should be for able-bodied people providing that assistance.

I'm so sick of seeing people with school-age kids trying to weasel their way in front of the F/MVPG line like the kid is some kind of golden ticket. Any parent who tries this should be required to carry the child all the way to the seat.