uh... well speaking from personal experience, I'm the middle of 5 and easily the most uh... "different-thinking" in my family. My ENFP brother is very non-conventional too, but when you want someone who's totally unshockable and will say the thing that makes everyone stare, it's me you come to. And I don't do it on purpose. In fact, I'm usually trying my hardest NOT to do it!! Oh, and my relatives also often remark on how they never count on being able to persuade or convince me of anything. My aunt put it this way "he'll listen to your points and take them on board, but as to whether it'll make any difference, you just can't ever tell. sometimes he'll totally give in and accommodate you, other times he won't budge an inch, and you can never really tell what it was with him that clinched it either way."
Of my kids, of which there are two, the oldest is very much the sheep, she'll do anything to fit in, and the youngest really couldn't give a shit, she will always just be herself and makes no apologies for it to anyone.
My sister has 4 kids and it's the second youngest who's the most independently minded.
So... my experience tends to say that in fact younger kids are more likely to buck against expectations based on elder siblings' performances, plus they're also less likely to HAVE so much expectation placed on them anyway, and so perhaps would feel freer to pursue unconventional avenues...