By the time you hit 7th grade I do believe you should be hitting -3. It is not like a kid is being judged by his power numbers in MS. by 14 many kids have already kicked in their growth spurts and handle -3 just fine. I have a small 14 year old who just one hopped a HS field with a shot to left with a -3. In competitive travel ball most 13/u teams can still use some sort of bat lighter than -3. Most travel teams have decently skilled players. Using a drop -8 in middle school teams where they still have players who are approaching the end of their baseball careers and not really moving beyond 7th or 8th grades is a bad idea. Imagine a big bodied skilled 13 year old using a drop 5 against a 7th grader who really is playing his last year of competitive baseball....Not good.
With out discussing wood, I personally believe the bat rules are fine where they at. Again, this is coming from father whose child has always been one of the smaller kids on any team hes played for.