A court heard James Nelson, 18, and another man targeted the vulnerable elderly victim in his sheltered accommodation.
Leeds Crown Court heard the victim suffers from dementia and has been targeted 16 times at his home in Rothwell in the past 12 months.
John Bull, prosecuting, said the victim answered a knock at the door at 2.15am on December 17 last year as he believed it was police officers who had come to check on his welfare.
Nelson then forced his way into the property as the other man stood in the doorway.
The teenager then grabbed the victim in a headlock in the bedroom before throwing him on to the bed.
Mr Bull said Nelson then went through the pensioner’s pockets and stole £20 despite his cries to leave him alone.
Nelson, of Swinnow Road, Pudsey, was linked to the office after he was caught on CCTV footage from a garage nearby around the time of the incident.
He pleaded guilty to robbery. He has two previous convictions for theft.
Michael Walsh, mitigating, said Nelson had not been responsible for the other offences committed against the victim.
He added that another is due to face trial for alleged offences against the pensioner.
Mr Walsh said his client had been naive for becoming involved and had been easily led by others.
The lawyer added: “He needs to learn and he needs to learn quickly.”
Judge Tom Bayliss, QC, said: “You and another man targeted him.
“You were not the ring leader, on him.
“ I accept that. But you went in to that house and you were the man who meted out violence.
“What marks this case out as very serious is the deliberate targeting of this man in a place where he and all of us have the right to feel safe and secure - in his own home.”