Have you found any indication in the text of who's older: Jon or Robb? Or even more specifically, is Jon presented by Ned to be older/younger than Robb, regardless of his true parentage? I haven't been able to put a cohesive timeline together in my head. Is Ned supposed to have sired him during the rebellion after leaving an already pregnant Cat?
It’s not made clear as far as I can tell, and I don’t think it will be entirely clear until Jon’s parentage is fully established. For example, if Ashara was Jon’s mother and he was conceived at Harrenhal, he’s older than Robb. That theory depends on Ned siring him prior to marrying Cat. If Wylla is his mother, however, he’s younger than Robb, conceived after Cat and Ned’s marriage. And if Lyanna is Jon’s mother, it’s hard to say whether Jon or Robb is older. They’d be born in the same year, and I wouldn’t be able to say exactly which earlier, because I think it would be very close.
Cat is concerned about the rights of her children, and I think perhaps if Jon was obviously even a bit older, that would have made her worry all the more. But we never have her mention that in her POV, so I wonder if that means infant!Jon looked younger.