Natural, generally. There are a few who were induced via a formula but that was... it's a long story.
For most, it's 'triggered' by puberty. Unless you're several generations in. Or you have a subtle ability that you don't notice until it's already been active for over a decade.
I know a six year old who's basically had his ability since birth. Third generation, at least. But he also has a shit father and a selfish cow of a mother. And I know what that can do to a kid so I've had to resist the urge to kidnap him a few times over the years.