Describing it to be "a serious type of offence", the judge said: "This man was very vulnerable due to his age and due to a medical condition."
"He was an elderly gentleman - you could have just walked away from him. Instead of that you assaulted him and humiliated him," the judge said.
The judge said that Singh was no threat to her and nothing could justify such an attack.
The girl's counsel, David Murray contended that Millerchip had "a very troubled childhood", the report said.
"Within both documents seen by the court you see a very fragile, vulnerable young person," Murray said.
Murray said Millerchip had shown real remorse since the attack.
Meanwhile, Singh's family in a statement said Singh was never the same after the brutal assault and he was "edgy" and "nervous" in his own home.
Millerchip was also jailed for 20 months for a burglary committed in July last year with brother Mason Millerchip and Jordan Arrol in a Coventry house where they stole a laptop and bank cards.
