His implication seems to be towards the historical childbearing age which skewed way younger, which is why shows like GoT wound up aging characters because framing the show within the context of historical accuracy makes modern viewers incredibly uncomfortable because the paradigm has gradually shifted.
That's a generally fallacious argument (riddled with bad faith assertions, including the fact that many girls who would have difficulty at early ages would *still* have difficulty at later ages, if they were able at all) that seems to rely more on modern social moors than actual biology. But at the same, I just don't care enough to break down where your arguments fall flat, particularly since I agree that in this day and age that preteens -- and teens in general (and even 18+ is iffy) -- shouldn't be getting pregnant within the current developed world. However, Sunny's assertion wasn't about the current developed world, so it doesn't line up there.
But from technical side, Sunny's argument reflects a general reality because we *have* seen stories about it, and he was discussing the idea from an absurd hypothetical stance that doesn't bear much relation to anything in our world today, which he was likely only doing to make an exaggerated point about our modern idea of adolescence (where adulthood has one from 14-16 to closer to 24 or 25, or even 30).
He ought to learn to articulate himself a little more thoroughly than relying on you to come out and white-knight him. He can defend himself like an adult instead of being a brat and saying 'I'm right, shut up, anyone refuting me is silencing me!'
Also, 'we *have* seen stories' represents a a far smaller statistic. You have the smarts to say 'we have seen' instead of 'we regularly see' . It does not mean 12 year olds are ready for pregnancy.