The suggestion was given to, "Think as far ahead as possible." and I want to emphasis that, further ahead in fact that you'd probably like to. If you can get differentiation enough to keep your child challenged at their current grade level I'd encourage doing so for multiple reasons...

1. Additional years in school which, with most GT kids, means more college credit courses taken in HS, meaning less to take in college and better prepared to meet the rigors of college (I know, thinking about college at this point is almost painful)

2. Graduation at an early age and then going to college at an early age only works well if a young person is emotionally ready and has the maturity to be able to handle life away on their own in college. A couple more years in HS taking college level courses not only lets the student gain more college credits at the expense of the HS rather than you (yes, I know the taxpayer flips the bill, that includes me too) but gives the student a chance to mature socially and emotionally. Once college comes around, more GT students probably fail because of either failure to be challenged in HS or social emotional reasons than anything else.