Vajra is somewhat cheating because the Blackstaff has part of Khelben's mind along with the minds of all the other Blackstaffs since him. And it is not only capable of teaching her (with its INT 22, WIS 18 and CHA 18), but it's actually smarter than she is. She's essentially a combination of natural talent plus being speed-taught by the staff, and the staff has a lot of experience to draw on.
That being said, if you start with characters who're young and a campaign only runs for a year or two of actual in-universe time, you'll potentially have 15th level 20 years olds (e.g. if you do Rise of Tiamat, for example).