Skakel, 51, was convicted in 2002 of the murder of Martha Moxley in 1975, when they were both 15 years old.
Skakel's lawyer, Hubert Santos, argued the 20-year sentence was excessive and asked for a reduction because his client would have received a maximum of four years in prison had he been tried as a juvenile, The Hartford (Conn.) Courant reported.
Prosecutor Susann Gill said the sentence was appropriate and Skakel could have been transferred to adult court and faced a longer sentence as a 15-year-old.
"He should have come forward and taken responsibility for this crime in 1975," Gill said.
The judges reviewing the case are expected to make a ruling on Skakel's sentence in two months, the newspaper said.
Skakel told the judges he is innocent of the crime, saying he passed three polygraph tests.
"I told Mrs. Moxley if I killed Martha I would take responsibility for it," Skakel said. "I didn't commit this crime."
Skakel's aunt, Ethel Kennedy, was married to Sen. Robert F. Kennedy, D-N.Y., who was assassinated in Los Angeles in 1968.