All children are heartless. They have not grown a heart yet, which is why they can climb tall trees and say shocking things and leap so very high that grown-up hearts flutter in terror. Hearts weigh quite a lot. That is why it takes so long to grow one. But, as in their reading and arithmetic and drawing, different children proceed at different speeds. (It is well known that reading quickens the growth of a heart like nothing else.) Some small ones are terrible and fey, Utterly Heartless. Some are dear and sweet and Hardly Heartless at all.
For though, as we have said, all children are heartless, this is not precisely true of teenagers. Teenage hearts are raw and new, fast and fierce, and they do not know their own strength. Neither do they know reason or restraint, and if you want to know the truth, a goodly number of grown-up hearts never learn it.