
There was a 17-yr-old that ran away from home because his parents were starving him. They had for most of his life. As such, he looked like he was only about 10 years old. When he arrived in Florida, the police were notified because people thought he was a little kid. The police took him into custody, found out what had happened and arrested the parents and although it took quite a long time, once he was getting regular meals and sunlight and nutrients, he FINALLY started to grow. And there have been MANY other situations like that. So yes, you have an extremely valid, and possibly correct theory, since it is documented to happen in real life.
Thus, we cannot say he is a child, a teen or an adult based solely on his looks.
Also, even though the "dad" called him a kid, that doesn't mean anything. My son is in his 20's and I still call him kid at times. I know other adults that do that as well. So we will just have to wait and see.

That must've been such a horrible and traumatizing experience for him. I don't know why parents give birth to a child if they are gonna treat them in such a disgusting way. You are a parent for a reason, you guide your kids to live a free life and let them make their own decisions, but also be there to warn them about vicious people. You support them, comfort them, and you will be there when they need help.
Some people aren't cut out to be parents, I guess.
Since we didn't get familiar with the characters, I'm not sure if what I'm saying is true, this is why I called it a theory.
I see a lot of people talking about the black-haired guy being a kid, even I'm unsure about it, and I don't want to defend pedophilia in any way.
What I thought about is the black-haired guy's past. We start by seeing how he was treated by his parents. He wasn't fed, taken care of, and not treated properly, etc. which can affect his growth, looks, weight, and health. That means he lacks a lot of vitamins and iron, to the point where he can't partake in activities. I'm not sure if he is anorexic, and I wouldn't be surprised if he is.
Anyway, these were just my thoughts on why he looks so small and frail