Emergency services discovered the boy suffering a gunshot wound at a unit in Norwest, in Sydney's Hills District, about 5pm on Monday.
The boy was shot from inside the home and was rushed to hospital in a critical condition.
Four 18-year-old men and a 44-year-old woman were also present at the luxury apartment complex when authorities arrived.
Three of the men were arrested, but two were released pending further investigation.
On Tuesday morning, police said one of the 18-year-olds had been charged with concealing a serious indictable offence.
He remains in custody to face Blacktown Local Court on Tuesday.
NSW Premier Chris Minns said he understood the adults who were in the house were not cooperating with police and called with anyone with information about the incident to come forward.
The shooting was a tragic reminder of the danger of guns, he said.
"Often the people who are the victims of guns are innocent people. Many times, they're children," he told ABC TV.
Superintendent Naomi Moore on Monday said the 11-year-old boy's mother was distressed by the incident and was by her son's bedside in hospital.
"She is as distressed, as any parent would be, about what she is dealing with at the moment," she told reporters.
"It is an (absolutely) tragic situation and as we've seen some tragedy across Sydney in the last couple of days, this is not far from the worst that I have seen."
Detectives had no reason to suspect the shooting related to organised crime, she said, but would not speculate about what occurred in the house.
Police established a crime scene at the address and an investigation is continuing.
Anyone with information has been urged to contact Crime Stoppers.
1800 RESPECT (1800 737 732)
Lifeline 13 11 14
Men's Referral Service 1300 766 491
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