A 90-year-old Indiana man who admitted serving as a cocaine courier for a Mexican drug cartel has been sentenced to three years in prison.

Leo Sharp, a decorated World War II combat veteran who turned 90 yesterday, was also sentenced to three years supervised release by Judge Nancy Edmunds in a US district court in Detroit.

Sharp told Judge Edmunds before she sentenced him: "All I can tell you, your honour, is that I'm really heartbroken that I did what I did. But it's done."

Afterwards, Sharp turned to prosecutors and called the three-year prison term a "death sentence".

Sharp was pulled over by police in October 2011 for erratic driving on an interstate highway in Michigan with what turned out to be 104 bricks of cocaine in his truck.

Prosecutors said Sharp hauled 1,250kg of cocaine into Michigan from the southwest US on six trips from February 2010 until his arrest, earning $1,000 per kg for drugs he transported.

He also hauled duffel bags stuffed with cash back to the southwest border of the US for the criminal organisation that was part of Mexico's Sinaloa cartel, prosecutors said.

He pleaded guilty last October to one count of conspiring to distribute cocaine in an agreement with prosecutors that included a recommendation for a prison sentence of five years.

Sharp's attorneys had asked for supervised release or home confinement, saying he suffers from dementia and other health conditions and needs 24-hour monitoring.

Prosecutors said, however, that Sharp had managed to avoid detection for a decade in part because of his age and was now trying to use his age to shield himself from punishment.

Judge Edmunds departed from both suggestions and said the effort to blame Sharp's actions on age and dementia was "an insult to all the people who struggle with dementia and don't become involved in illegal activity".

Judge Edmunds said Sharp was in the middle of the drug conspiracy.

"This is not a victimless crime", the judge said. "This is a very serious offence."

Sharp's plea agreement and reduced charge allowed him to avoid a mandatory ten-year minimum sentence.

He agreed to forfeit two properties in Florida and $500,000 cash, but not his home and farm in Michigan City, Indiana.