The 19-year-old, who was on temporary leave from a treatment centre at the time of the attack, was convicted for murder and could have been sentenced to life in prison.
But the district court in Skaraborg District Court reduced the man's sentence in light of his young age, reducing the prison term to nine years, according to TV4.
The incident took place the evening of March 21st when the 19-year-old knocked on the door of an elderly couple in Skövde in central Sweden.
When the 86-year-old man opened the door, the 19-year-old gave him a vicious beating. The man fell into a coma and died five weeks later.
During the trial, the 19-year-old said he was intoxicated during the attack and only has fragmented memories of the incident.
An investigation of the 19-year-old following the crime revealed that he suffered from ADHD and a mild form of autism.
While he confessed to serious assault and manslaughter, the 19-year-old didn't believe he had committed murder, but the court ultimately ruled otherwise.