The 17-year-old boy killed Frank Sinclair in a brutal attack which took place in East Kilbride in January last year.
The youth, who cannot be named for legal reasons, preyed on Mr Sinclair because he believed the pensioner had earlier pushed him outside a community hall.
The schoolboy scraped his face off a wall at the complex and this caused him to attack Mr Sinclair.
The High Court in Glasgow heard how the boy and three schoolgirls found Mr Sinclair lying on the ground at the rear of the community centre at around 8pm.
Following an incident, the boy repeatedly stamped on his head.
A pathologist told jurors that the blows were so intense that Mr Sinclair’s spine had been severed.
At least 11 years in custody
At the High Court in Edinburgh today (Thursday), judge Lord Burns told the boy that he’d have to serve at least 11 years in custody.
Telling the boy that he’d only be released when the parole board were satisfied that he no longer posed a threat to public safety, Lord Burns added: “You by your actions killed Mr Sinclair in an attack which was so far as I can see wholly unprovoked by him.
“You proceeded to stamp on a vulnerable and immobile 61-year-old man who you had encountered lying on the ground.
“By this terrible act you killed him and deprived his family and friends of his love and companionship and they will have to live with that for the rest of their lives and you have to live with that for the rest of your life.
“You acted as you did so it is the most serious of crime. You fled the scene knowing Mr Sinclair had been injured.”
The boy was convicted of murder following a trial last month.
He was also found guilty of an unprovoked attack on an 18-year-old man by punching him on the head and body at Westwood Stores, in East Kilbride’s Leeward Circle.
At proceedings last month, a teenager told the court how she, two other girls and the boy found Mr Sinclair lying on the ground.
She said the boy tried to help Mr Sinclair by lifting him up from the ground, but both fell and the boy injured his face, scraping it off the side of the building.
One of the girls phoned for an ambulance and the teenage boy walked away.
The court then heard how the boy returned about 15 minutes later with the witness adding: “He was more annoyed than he left.”
The girl, who is also 17, said one of her girlfriends grabbed the boy and tried to hold him back but failed.
The girl, who also can’t be identified, said: “He ran up to Mr Sinclair, raised his foot and put it down on his face.
“I could hear the sound of his foot hitting the head - the stamps were hard and forceful.”
Pathologist Sharon Calvert told the court that Mr Sinclair had been “internally decapitated” after having his head repeatedly stamped on.
She said Mr Sinclair had 19 injuries to his face and neck including a fractured hyoid bone.
An internal examination showed that a vertebrae at the top of the spine near the larynx was fractured.
Mr Sinclair’s friend David Henderson,50, said he had been with him earlier in the evening and had enjoyed some drinks.
He said that he left Mr Sinclair at around 6pm and his pal was sitting on the steps of the community centre rolling a cigarette.
Admitted to the attack
The teenager gave evidence during proceedings. He admitted to prosecution lawyer Liam Ewing QC that he attacked Mr Sinclair.
When Mr Ewing asked what his intention was as Mr Sinclair lay on the ground, the teenager replied: “To hurt him.”
He added: “I just lost my temper. I just lost it. I thought he’d be knocked out.
“I didn’t know what I did to him would kill him.”
On Thursday, defence advocate Bert Kerrigan QC told Lord Burns that his client came from a stable background and a loving family.
However, the court heard the accused had anger management issues and had a “propensity to resort to violence.”
Mr Kerrigan added: “It is a sad feature.”
Mr Kerrigan also told the court that his client had expressed remorse for his actions.
He added: “I would ask your lordship to exercise a degree of leniency and to take his age and his contrition into account when passing sentence.”
Lord Burns also sentenced the boy to two years to in relation to the assault charge. He ordered this sentence to be served concurrently with the 11 year punishment part.
Lord Burns added: “I must pass sentence of detention without limit of time.”