The Daughter of Etaret Witchbane, Astrid grew up in Windhelm trained in magic by the Templars and her 'uncle' Gaius specifically. She is a gifted mage and got accepted into the College of Winterhold at 17.
Astrid Steelfang, born Astrid Witchbane, grew up in Windhelm. As the daughter of the Jarl she was slightly spoiled getting nearly everything she wanted. She had a pretty uneventful life as a child. When her father learned of her skill in magic he had Gaius and Wuunferth train her in various schools of magic.
When Astrid was seventeen she enrolled in the College of Winterhold, wanting to learn magics that the Templars didn't use such as necromancy and mind magic. She passed initiation and was admitted in. She quickly excelled at all the basic magics, being a fast learner, but she didn't really gain power until two years after enrolling when she met Raydin Steelfang. Raydin took on Astrid as an apprentice and privately tutored her.
During this time Adawulf began tormenting Astrid with visions of her father's death and the destruction of Skyrim. She went to Gaius for healing but he couldn't as nothing was actually damaged. He advised her to go to the Archmage to learn to defend her mind as he couldn't quickly teach her a way to block someone as powerful as Adawulf.