I tend to agree with Crisc as well. A PG child does not always start college exactly when they are ready. If they are kept at grade level, they actually may be disengaged from all schooling and at a great risk for being a drop out. I read some interesting statistics about all gifted children and their HS drop out rates. If a child is in public school, parents have to advocate constantly and do a considerable amount of afterschooling.
Also some EG and PG children have a very narrow focus of interest and tend to submerge themselves into this topic. They may not have the motivation to excel in other subjects the same way. They may have the cognitive readiness to absorb indefinite amount of concepts but not the motivation.
I am curious where Ruf sees the cutoff btw. EG and PG as it relates to college? I understand IQ differences but I would think with college so many external factors come to play. I also think the EG/PG lumping occurs when the number of PG kids is statistically small and while there are many differences, there are similar characteristics.