"He repeated seventh grade four times before he failed to proceed past seventh grade," Berryman said.
Assistant District Attorney Ashley Lynam asked Woelpper to impose a prison term of 57 to 114 years, saying Walker "deliberately chose to prey on the most vulnerable members of our society."
Berryman had suggested a 15- to 30-year sentence and said Lynam "was asking for a life sentence. I don't think that's what called for here."
Lynam said that between July 14 and July 26, 2012, Walker beat and sexually assaulted four women - ages 42 to 54 - three of whom lived in group homes within three blocks of his house in Tioga.
Each woman was lured into a vacant building or alleyway and then beaten into submission before being raped, Lynam said.
The intellectual disabilities of each victim were so profound that they were unable to understand the danger of following a stranger, Lynam added.
Two victims made brief victim-impact statements describing their pain from the assaults.
"I have difficulty sleeping," said the 42-year-old victim of a July 21 assault in which Walker lured her into an abandoned house to "have a smoke."
Lynam said Walker had raped the same woman three weeks earlier and had no remorse. In one police interview, she added, Walker "analogized rape to finessing a woman."
Walker's sister, Sharonda, testified on her brother's behalf about his problems learning in school and a childhood in which he was beaten unconscious by their father.
"I love my brother to death," she testified. "I'd trust him with my life."