Jonty Bravery, now 18, attempted to enter the UK’s tallest skyscraper before heading to the London art gallery on 4 August last year.
Prosecutor Deanna Heer told the Old Bailey he travelled to the Shard, next to London Bridge station on the banks of the River Thames, and asked about buying a ticket for its viewing area.
Ms Heer said he did not have enough money and added: “He was later to admit that he was asking where the next highest building was.”
Bravery walked to the Tate Modern, which is free to visit, and was seen looking over the railings of the 10th floor viewing platform and ”behaving in an unusual way“.
The victim and his parents, who were French tourists on holiday in the UK, entered the area minutes later.
CCTV footage showed him turning towards the victim’s family as the boy skipped a little way away from his parents.
”As [the boy] approached, the defendant scooped him up and, without any hesitation, carried him straight to the railings and threw him over,” said prosecutor Deanna Heer.
“The CCTV footage shows him falling head-first towards the ground.”
Bravery, then 17, was seen backing away from the railings while smiling and appearing to laugh immediately afterwards.
When challenged by the victim’s father, Bravery told him he was “mad” and witnesses described him having a “big smile on his face”.
The teenager, from Ealing in west London, then sought out Tate staff and confessed: “I think I’ve murdered someone, I’ve just thrown someone off the balcony.”
The boy fell five storeys before landing on a roof, and was left with life-changing injuries.
He remains in a wheelchair with constant care, and it is not known whether he will make a full recovery.
Ms Heer said the victim was “fortunate not to die”, adding: “This was a whisper away from a murder.”
The court heard Bravery had planned the offence “well in advance” and researched the easiest way to kill someone.
Ms Heer said the teenager conducted a variety of searches including, “are you guaranteed to escape prison if you have autism?”, “what are the chances of death if you push into the River Thames?” and a web page entitled “how to get away with rape.”
The prosecutor added: “He narrowed it down to three possibilities: strangling a woman or a child, drowning a child, or throwing someone off a tall building.”
Bravery told medical experts he felt “indestructible” and “on top of the world” after throwing the boy from the viewing platform.
He was under the care of Hammersmith and Fulham Social Services at the time, and living in supported accommodation.
Bravery was described as abusive and aggressive towards staff, and assaulted a female care worker in 2017.
At the time of the attempted murder, he was under one-on-one supervision but was allowed to go out unaccompanied for four-hour periods.
The court heard that Bravery has autism spectrum disorder and a personality disorder, both of which are relevant to understand his behaviour.
A recording, taken by one of Bravery’s care workers in autumn 2018, suggested opportunities to stop the attack were missed.
“In the next few months I’ve got it in my head I’ve got to kill somebody,” he could be heard saying, describing his urge to push someone off a building in central London.
“It could be the Shard, it could be anything just as long as it’s a high thing and we can go up and visit it and then push somebody off it and I know for a fact they’ll die from falling from a hundred feet.”
The former care worker who took the recording told the BBC that Bravery had mentioned similar plans several times, adding: “There were a few incidences regarding trying to hurt people, life-wrecking incidences that he had planned in his head.”
In a victim impact statement, the victim’s parents said they had not felt able to leave their son’s side for more than a couple of hours because they are “so scared of losing him”.
“The act committed by this defendant against our son is unspeakable,” they added, saying their son was unable to trust people and “would like to slap” Bravery for what he did.
Bravery pleaded guilty to attempted murder in December.
He sat with his T-shirt pulled over his head, then crouched on the floor with his back to a videolink camera as details of the case were read out at Thursday’s sentencing hearing.
Bravery said that he had been “seriously unhappy” and “hearing voices” telling him to hurt and kill people for months before the attack.
He told police he had to prove a point “to every idiot” who said he had no mental health problems, saying he wanted to be on the news “so when it is official no one can say anything else”.
In a series of social media posts before the attack, which have since been deleted, his father Piers Bravery attempted to raise awareness of autism and urged the health secretary to “do your job and stop more children dying”.
Bravery is currently being held in a secure unit at Broadmoor Hospital.
The judge, Mrs Justice McGowan, said she would not sentence him until Friday morning, adding: “It is obviously not a straightforward case.’’
Additional reporting by PA
Register for free to continue reading
Registration is a free and easy way to support our truly independent journalism
By registering, you will also enjoy limited access to Premium articles, exclusive newsletters, commenting, and virtual events with our leading journalists
Already have an account? sign in