Have you seen his content? Comparing Pewdiepie's behavior to that of a teenager is an apt comparison and reflective of his primary audience. My contention is that he has never grown out of that mindset. With his success as a young adult, he's still never had a reason to really grow up. It doesn't matter when his birthday is if he never intellectually and emotionally developed beyond that.
For me, if he re-uses the word in his inevitable follow-up video in a defiant manner that is not just to clarify the word he said prior to including an apology, and then proceeds to defend his actions with the assertion he will or would do it again, that would be firmly indicative of someone with overt racist tendencies and sympathies. I do not think simply labeling people as "racist" and moving on is any sort of answer. It's not as important as understanding what the actual motivation for their words really is. If it comes from a place of ignorance, that is one thing to me. If it comes from a place of hatred, that is another. I don't feel satisfied relying on blanket terms to decide how I feel about a specific person's personality or actions or history.