I went to a gifted HS and the "normal" entry was to skip a grade when coming in (at 8th grade essentially). All my sibs did this (2 older, 1 younger). I did not. Now don't get me wrong, they are all very happy, successful, socially adjusted adults, but where it seemed to make a difference (to my view) was upon entering college. Being so young in college seemed to put some strains on them that I'm not even sure I could articulate - I just could feel it was something harder for them.

In my case, I didn't skip the grade, and I also took a year before college, which I spent in France, attending French HS and living with a family. So I was an 'old' freshman, and I loved having a more mature outlook that had come from both age and having already been away from home on my own in a different country.

So, I lean toward the earlier sentiment that it never seemed a disadvantage to be the older one, but it could hurt to be the youngest one.