1. His brother is as young as my brother.
2. His borther is as old as my brother.
the more I thought, maybe they do mean the same thing? The more I babble like below, the more they all sound the same, or equally correct, to me...
"Hey, Dennis, doesn't Tommy have a little brother like you do?"
"Yeah, his brother is as young as mine, just a little guy."
"And doesn't he have a little sister, too?"
"Yup, she's as old as my little sister is... they're both six."
"Oh. How old is your little brother?"
"Oh. And your other little brother looks tiny. How young is he?"
"He's only five months."
I might be wrong, but aren't old and young examples of unmarked words?
Like "long", in its marked form, means something of substantial length... but in its unmarked form, it refers merely to length, even if it is short.
The ship is very long. (Marked)
How long is the amoeba? (Unmarked)
The boy is old.. he is an older brother... both marked.
How old is the boy? How old is your baby brother? Seven days. Unmarked form.
The boy is young.. how young is your brother? Twenty-three.
Oh. How old is your other brother? Nineteen.
Though, like always, I could be on a completely wrong track with this... I haven't even thought about marked and unmarked words for a looooong time, to use a marked word...
Thank you very much, Robert, for the very interesting answer.
"I am an old man." (Marked) "I am 70 years young." (Unmarked)
"He is a young boy." (Marked) "He is seven years young." (Unmarked)
Do they work?
I could be on a completely wrong track with this... I haven't even thought about marked and unmarked words for a looooong time, to use a marked word...
Not at all. Markedness is important . . . given the right semantic context as you've shown in your examples. As is though,  and  express different things, the default reading. Provide additional context, and the meaning changes, evolves. Environment is everything.