He grew up being well traveled, diciplined, educated, and even experienced a pre Aku world. Jack is well traveled warrior who has been alive in this future for say about 54 years (year per season plus the 50 gap). He was not only physically stunted but he maintained the mind/experience/maturity of a 12 year old being trapped in ice and all.
It's 100% forced with little foreshadowing till she scrubbed all the soot off.
