In the book, there’s a character named Brady who is a seventeen year old kid in the body of a six year old. In part, this was just to illustrate that there’s some side effects to having a pregnancy partially out in the wastelands, but there’s an actual rare condition that has occurred that is related.
Brooke Greenberg was diagnosed with something called “Syndrome X” which meant that aged incredibly slowly. It’s a bit of a medical mystery, with only a handful of people developing it worldwide at any given time. Interestingly, their mental states also don’t age, which means it’s not a glandular problem like in some cases, but that they actually aren’t developing at the speed of other people.
I gave Brady a modified version of it. In the universe of After Destiny, there is a treatment for it, though they had a very limited supply of it and it only allowed Brady to age until the point he is now physically. I had plans to do more with it, but it didn’t fit within the context of the revised narrative and, like many other little darlings, the full explanation of Brady’s medical condition was abbreviated and largely cut back.
If you get a chance, though, do read up on Brooke Greenberg and Syndrome X. It’s fascinating and I couldn’t help but draw a little inspiration from it.