Here's a not so official commentary on the average IB score you should aim for. Please don't take it seriously. Remember applying to US universities is not solely score based but a holistic process that takes into regard your recommendations, your extracurriculars and essays.
"At least for the Ivy League and more selective colleges, you’re going to need a healthy mix of sixes and sevens (no less than six) in your subjects for a shot at admission.
A score in the 40s is going to make you a more competitive candidate academically (note: a lot of non-academic factors are at play), but a 38 is considered a good grade.
If you’re aiming for the Ivies, you certainly don’t want to be dipping below the 36 mark. Even then, the rest of your application had better be pretty exceptional to compensate for a score on the lower end. "