Aptitude for Busoshoku? My question is about busoshoku haki or coat of arms as people like to call it. Until the last episode I thought haki was something that required intensive training to master. But in the last episode 

For those who haven't watched spoilers:


  Sabo was shown wielding busoshoku against a small army at 17. 


Now I only started reading the manga in the zou arc so I'm not sure if this is filler or cannon but is it safe to assume then that some people just have an aptitude for mastering buso quickly? I mean only a small percentage of people are capable of using haoshoku so it seems like it makes sense. 

But it took Luffy two years to perfect it and I'm not even sure if Ace ever learned it. Does anyone have any feedback on this?
Haki does require intensive training to master, but after being rescued by Dragon at the age of ten, he presumably started training with the revolutionary army shortly after his injuries healed.

At this point, Sabo wasn't just your average ten year old, having been fighting Luffy and Ace many times every day, he was already stronger than an average pirate or soldier. It seems likely that Dragon or Ivankov or another high ranking revolutionary saw his potential, and trained him specially, as by just the age of 22 he had already risen to the rank of Chief of Staff, number two in the revolutionary army. 

Unlike Luffy, he was in a position where he was surrounded by strong allies who could train him and teach him useful skills like Haki and martial arts, so having learnt to use busoshoku haki effectively by seventeen isn't hard to believe. Except for in the New World, haki is a very rare skill, and considering it took Rayleigh eighteen months to teach Luffy the basics. Luffy never had anyone to teach him growing up in Fusha Village and then setting sail by himself, as you would need someone who knows haki themselves, which is rare, and is also willing to spend the time to train you.