User blog comment:Ero-cook/Why didn't Ace get to learn Busoshoku Haki?/@comment-2031609-20110704160852

I agree with Jinbe, it takes a long time to learn and master it. Luffy has possibly mastered it is because he spent almost every moment of two years training it, he has the ability to learn things involving fighting fairly quickly, as can be shown with his adaption of Soru, and the fact that he had a master for a teacher. Plus, though Ace was with the Whitebeards for seemingly most of the 3 years between when he set out and Luffy set out, for a time he had his own crew and even after joining the Whitebeards he was still very cautious, then having to leave for a time to search for Teach. During that period Ace was probably not even considering haki, but learning his DF abilities and managing a crew. Even if he figured out how to do some of haki, the Whitebeards could only teach him so much, and he only had a limited time to learn it, as they were constantly in battles and what not. Had this happened maybe 2 years later, I think Ace could have learned and used it, but as that is not the case, then he probably simply did not have adequate time to properly learn it.