So far, there have been only two captains named as teenagers, Connor McDavid and Vinny Lecavalier. On top of this, Sidney Crosby, Gabriel Landeskog, and Jonathan Toews were all captains at 20.
First off, naming a player a captain before they even play a game for your franchise is normally reserved for well-established NHLers like John Tavares. These players tend to be big-time free agency signings and already have captain experience with other NHL teams.
On top of this, the Blackhawks have an all-star in Seth Jones who certainly could be their captain himself. It would likely be a slap in the face to Jones to name a teenager who has not even stepped foot onto NHL ice the captain before him.