One might argue that a cultivator isn't born with the knowledge of how to cultivate Qi, he learns it as he grows older and for about ten years, he doesn't cultivate because of his very young age and weak meridians.
But this fact doesn't mean there's no Qi in his body, in fact, the body of a cultivator is naturally sensitive towards Qi and automatically absorbs it even though it doesn't make him cultivate or increase his cultivation.
There's always a little flow of Qi even through the body of a toddler. Ye Tian Yun blew out air from his mouth as the pain slowly subsided.
But he knew it wasn't gone for good, but rather it was only stopped because the needle was not moving. As soon as he touched his acupuncture needle again, the pain would start once again, and as he got closer to the meridian, it would only increase more in terms of intensity.