Former U.S. House Speaker Dennis Hastert, once one of the country's most powerful politicians, was sentenced on Wednesday to 15 months in federal prison for a financial crime related to his sexual abuse of high school wrestlers decades ago.

U.S. District Court Judge Thomas Durkin called Hastert a serial sex abuser in handing down the sentence, which was longer than the zero to six months recommended by federal prosecutors. Durkin said the sentence would have been even longer if it weren't for Hastert's age, 74, and poor health.

Frail and clinging to a walker, Hastert acknowledged he sexually abused boys when he was a teacher and coach in his hometown of Yorkville, Illinois and apologized.

"I'm sorry to those I hurt and misled. I wanted to apologize for the boys I mistreated," Hastert told Durkin at the two-hour sentencing hearing in federal court in Chicago.

Durkin asked Hastert if he acknowledged sexual abuse of his victims and Hastert said he did.

Hastert, the longest-serving Republican House speaker in history and a successful international lobbyist, last October pleaded guilty to structuring, which is withdrawing a large sum of money in small increments to avoid detection.

He needed the cash after he reached a secret agreement with one of his five victims to pay him $3.5 million in compensation for pain and suffering.

The judge said that even though Hastert could not be charged with sex abuse because the statute of limitations had run out, he could take the conduct into consideration at sentencing.

"Some conduct is unforgivable no matter how old it is. The abuse was 40 years ago but the damage lasts today," Durkin said.

Durkin said that an extremely aggravating factor in the sentencing was that Hastert lied to federal agents about the money and falsely claimed that the victim was extorting him. He said Hastert must serve two years of probation and go through sex-offender treatment after release, and also pay a $250,000 fine.

Scott Cross, 53, one of Hastert's victims, spoke during the hearing. It was the first time that any of his victims has spoken publicly.

"I wanted you to know and understand how Mr. Hastert violated the trust I put in him as a high school student.

I wanted you to know the pain he caused me then and still does today," Cross told Durkin, as he choked up during his declaration. He said Hastert abused him in the wrestling locker room in 1972 when he was 17 years old.

Hastert, a conservative who trumpeted his honesty and small-town values when he was in Washington, D.C., said he did not contest Cross's statement.

Hastert's defense said the former politician had forgotten the incident from 1972 and had even approached Cross's older brother, Tom Cross, former Illinois House Republican leader, and asked him to write a letter of support to the judge.

The sex abuse victim who received money from Hastert has still not been named publicly. On Monday, using the name James Doe, he sued Hastert for $1.8 million, saying he had received only $1.7 million of the promised compensation.

Hastert had been out on bail pending sentencing. He must report to a federal facility and will be sent to a prison where he can get medical treatment after suffering a life-threatening blood infection last year.

Barbara Blaine, national president of the Survivors Network of those Abused by Priests, a group that advocates for sex abuse victims, said she was pleased with the sentence.

"It will ... help encourage others who saw, suspected or suffered child sex crimes to protect kids by calling police, exposing wrongdoers and seeking justice," Blaine said in a statement.

(Reporting by Fiona Ortiz; Editing by Bill Trott and James Dalgleish)