Prosecutors have asked a US judge to sentence Bill Cosby to five to 10 years in prison for drugging and sexually assaulting a woman.

The defence argued that the 81-year-old comedian is too old and helpless to do time behind bars.

“What does an 81-year-old man do in prison?” defence lawyer Joseph Green asked on day one of the sentencing hearing for Cosby, who is legally blind and dependent on others.

“How does he fight off the people who are trying to extort him, or walk to the mess hall?”

Mr Green suggested that Cosby instead be put on something akin to house arrest.

Montgomery County District Attorney Kevin Steele said he has no doubt Cosby would commit another such offence if given the opportunity, warning that the TV star seemingly gets a sexual thrill out of slipping women drugs and assaulting them.

“So to say that he’s too old to do that, to say that he should get a pass, because it’s taken this long to catch up to what he’s done?” Mr Steele said, his voice rising.

Bill Cosby
Bill Cosby accuser Andrea Constand leaves the courtroom (David Maialetti/AP)

“What they’re asking for is a ‘get out of jail free’ card.”

And he said the sentence should send a message to others.

“Despite bullying tactics, despite PR teams and other folks trying to change the optics, as one lawyer for the defence put it, the bottom line is that nobody’s above the law. Nobody,” the district attorney said.

Judge Steven O’Neill is expected to sentence Cosby on Tuesday.

The TV star once known as America’s Dad for his starring role in The Cosby Show could become the first celebrity of the #MeToo era to be sent to prison.

Cosby was convicted in April of violating former Temple University women’s basketball administrator Andrea Constand at his suburban Philadelphia mansion in 2004.

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Former model and Bill Cosby accuser Janice Dickinson was in court (David Maialetti/AP)

After giving evidence for several hours at two trials, the first of which ended in a hung jury, Ms Constand spoke in court on Monday for just two minutes.

“The jury heard me. Mr Cosby heard me. Now all I am asking for is justice as the court sees fit,” said Ms Constand, who submitted a much longer victim-impact statement that was not read in court.

Mr Steele quoted Ms Constand in her statement as saying that Cosby took “my beautiful, healthy, young spirit and crushed it”.

The three charges on which Cosby was convicted carry up to 10 years in prison each, but both sides agreed to merge them together for sentencing because they stemmed from the same encounter.

State sentencing guidelines call for about one to four years behind bars on the combined charge.

The judge is also expected to decide whether to declare Cosby a “sexually violent predator”, a scarlet letter that would make him subject to mandatory lifetime counselling and community notification of his whereabouts.

On Monday, Kristen Dudley, a psychologist for the state of Pennsylvania, said that Cosby has an uncontrollable urge to violate young women and would probably commit another such offence if given the chance.

A psychologist for Cosby’s side is set to give evidence on Tuesday.

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Bill Cosby departs after a sentencing hearing (Matt Slocum/AP)

Cosby’s lawyers argued that the state law on classifying sexual predators is unconstitutional.

They contended also that Cosby is unlikely to commit another crime because of his age and health and because there have been no complaints that he molested anyone in the 14 years since his encounter with Constand.

“The suggestion that Mr Cosby is dangerous is not supported by anything other than the frenzy,” Mr Green said, alluding to protesters outside the courthouse and public debate about the case.

Ms Constand’s mother, Gianna, also went in the witness boxy and attributed her health problems to Cosby-related stress.

She accused Cosby of “ruining many lives.”

“I can only hope and pray that some sense of peace and faith can be restored back on our family,” she said.

“The victims cannot be un-raped. Unfortunately, all we can do is hold the perpetrator accountable.”

Cosby, looking grim, walked into the courthouse in the morning on the arm of his longtime spokesman as protesters shouted at him.

His wife of 54 years, Camille, was not in court.

Cosby spokesman Andrew Wyatt told reporters in the afternoon that the entertainer was in “great, great” spirits.

“We tell him to stay strong and stay focused, and he’s focused on Mrs Cosby, and that’s what matters in his family,” Mr Wyatt said.

Photo Gallery Bill Cosby
Bill Cosby pictured in 1972 (AP)

“He’s a great guy. He’s still America’s Dad, and they won’t ever take that away.

“You can’t take away the legacy.”

In the years since Constand first went to police in 2005, more than 60 women have accused Cosby of sexual misconduct, though none of those claims have led to criminal charges.

At least two of those women, Lise-Lotte Lublin and former model Janice Dickinson, were among those in the courtroom for the start of the sentencing.

Prosecutors had hoped to have some of the other accusers address the court at sentencing.

But the district attorney’s office said that that would not happen.

A few hours before the hearing, Ms Constand tweeted Ephesians 4:26, a Bible verse about letting go of anger: “Be wrathful, but do not sin; do not let the sun set while you are still angry; do not give the devil an opportunity.”

Cosby, who grew up in public housing in Philadelphia, became the first black actor to star in a prime-time TV show, I Spy, in 1965.

He remained a Hollywood A-lister for much of the next half-century, hitting his peak in the 1980s with the top-rated Cosby Show as the warm, wisecracking father, Dr Cliff Huxtable.

Ms Constand and other accusers have waived their anonymity.