As a 10-year old, my introduction to cryptography was from reading a few novels by Jules Verne:

The Giant Raft or 800 Leagues On The Amazon

Mathias Sandorf

A Journey To The Centre of the Earth

After reading those, I played with the ciphers described there, i.e. I made and used the transposition grid from Mathias Sandorf and I used various Vigenère ciphers, like that from "800 Leagues On The Amazon".

Due to the interest generated by those stories, a few years later I read various textbooks about modern cryptography.

The book "Applied Cryptography" by Bruce Schneier is now somewhat old, so it is no longer up-to-date, but it is written in a style more accessible for newbies than most textbooks about cryptography, so I think that it would be adequate for a teenager who is good at mathematics.