Aptitude for Busoshoku?
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.