Here's what I don't get. Doesn't Australian government documents show him as 19 years old? They would have to, since they would have been used to obtain the Canadian visa.
So if he lied about his age, it would have been when he moved from Sudan to Australia. His documentation regarding his age would have then been used to put him in an age appropriate class. If the docs showed him to be 13, he would not have placed in a class with 17 year olds. Couldn't it be possible that the year book lists all students, not just seniors?
I'm not saying he's definitely not lying about his age, but the yearbook story doesn't make any sense.
I don't know anything about Australia, Sudan refugees, immigration, etc. But the way I imagine it went (if any of this is true):
Thon Maker arrives in Perth as a 13-14 year old. Here's this refugee straight from Sudan with no documentation, but we're telling you he's 13-14 years old. Ok, here's his documentation. Then he goes to high school for 4 years in Perth.
Now shady basketball scout finds him. Moves him across the country to Sydney (this is like LA to New York distance wise). Says here's this refugee straight from Sudan, he's 13-14 years old but he has no documentation. Ok, here's new documentation for him.
Or shady basketball scout just greased a few palms. Both Sudan and Uganda where Maker grew up are extremely corrupt and prone to bribes.
Not knowing anything about the actual process, either seems plausible to me at this time.