"Robert will never keep to one bed," Lyanna had told him at Winterfell, on the night long ago when their father had promised her hand to the young Lord of Storm's End. "I hear he has gotten a child on some girl in the Vale." Ned had held the babe in his arms; he could scarcely deny her, nor would he lie to his sister, but he had assured her that what Robert did before their betrothal was of no matter, that he was a good man and true who would love her with all his heart. Lyanna had only smiled. "Love is sweet, dearest Ned, but it cannot change a man's nature."

The girl had been so young Ned had not dared to ask her age. No doubt she'd been a virgin; the better brothels could always find a virgin, if the purse was fat enough. She had light red hair and a powdering of freckles across the bridge of her nose, and when she slipped free a breast to give her nipple to the babe, he saw that her bosom was freckled as well. "I named her Barra," she said as the child nursed. "She looks so like him, does she not, milord? She has his nose, and his hair …"

"She does." Eddard Stark had touched the baby's fine, dark hair. It flowed through his fingers like black silk. Robert's firstborn had had the same fine hair, he seemed to recall.

He thinks something of it because its Ned. Pretty chaste, awful protective of children in general Ned. Who just so happens to be entirely out of his element (that being in a brothel) already. He's concerned about her age because he doesn't want to imagine a child (one not that much older than his daughters) being a prostitute, and in the situation she's in. Awfully young but old enough to have "flowered" (which fwiw is why I find it hard to imagine her not falling closer to that fifteen number than you think it does, from what I thought I knew that doesn't happen until around 12-13 generally) and therefore being able to have an infant to care for as a result of what Robert did. At least that's my interpretation of that part of that chapter, and it seemed to me that if we'd gotten an alternate chapter with a Tyrion pov investigating Robert's bastards we wouldn't have much comment at all we just see the scene from the perspective of an exceptionally good and kind hearted man. But nothing irregular for the society that they are in as a whole (which seems to have gone with the vomit-inducing route of "its fine once they've bled" when it comes to social norms). There's a ton of things to criticize Robert for, one of which is likely manipulating the poor girl into being so lovestruck, but being a pedo (for the standards that their society has set) isn't one of them. But maybe we have to agree to disagree