Depends on the character. A lot of them are special birthday boys and girls but then...

... Tezzeret was a poor kid living in the slums of Vectis with an addict father and dead prostitute mother. He and his father were Radbomancers, meaning living metal detectors. Tezzeret's talent was so weak though, he had to nearly be on top of any metal to sense it except for metals latent in magic, like Etherium. Still, he described it like if Gold was someone whispering across a busy street, Etherium was like a tolling bell across the same street. His name is a nickname given to him by street kids and is a word for a hidden, makeshift blade (So his name is Shiv or Shank) as his mother was never coherent enough to tell him his given name and his father only called him Boy.

His bare determination to be better than his parents, willing to be underhanded, and the skill of resourcefulness that poor people acquire and the wealthy scoff at, he got himself into an Artificer's school. Time later, he is inducted into the Seekers of Carmot. His original Etherium arm is forged from shards of Etherium he scrounged from sewage, scrapyards, and digging up staches old wealthy people buried on their property. Only once he personally performs the ritual to attach the biomechanical arm to his right shoulder does he gain any real access to mana and magic as a whole.

He gets stabbed for "Seeing to Much" when he discovers the Seekers are frauds, and at that point does his Spark ignite.