New York ? To senior citizens’ advocates, Brooke Astor is a Park Avenue poster child for an insidious kind of financial crime.

They kept close tabs as the late philanthropist’s son and a lawyer were tried on charges of exploiting her mental decline to raid her nearly $200 million fortune. An article on the AARP’s news Web site called it “the most infamous case of financial elder abuse in recent memory.”

Advocates and legal experts saw last week’s convictions as a high-wattage signal that such cases, often seen as difficult to prosecute, can succeed — even if few others spur a five-month-long big money trial with boldface names.

“To lose this kind of case would have sent a very discouraging signal” to prosecutors pursuing elder abuse cases, said Thomas L. Hafemeister, a University of Virginia law professor who specializes in financial exploitation of the elderly.

There have been plenty of prominent court fights over claims that elderly millionaires were manipulated into parting with money.

J. Seward Johnson Sr.’s children accused his third wife — and former chambermaid — of browbeating the dying drug company heir into leaving her nearly all his $500 million fortune; the 16-week trial in 1986 ended with a settlement giving the children and an oceanographic institute about $160 million. Former Playboy Playmate Anna Nicole Smith’s inheritance tussle with her oil-tycoon husband’s son reached all the way to the U.S. Supreme Court but continues years after both she and the son died.

But these and many other fortune feuds played out in civil courts — not in criminal cases carrying the prospect of prison time, which Astor’s 85-year-old son now faces.

Criminal cases in which ailing elderly people are conned by identity theft, real estate scams or light-fingered caregivers are fairly common, although few are as extensive as the Astor case — where prosecutors used thousands of pages of documents and called dozens of witnesses ranging from Henry Kissinger to household helpers.

Some prosecutors focus on scenarios involving physical harm. Others are reluctant to take on cases that can be blurred by family loyalties and disputes, said Catherine T. Wettlaufer, a Buffalo estates lawyer.

The cases also can be hard to prove. While there may be a financial paper trail, the victims often are unable to testify or unwilling to take the stand against a relative or needed caretaker.

Prosecutors then have to prove they know what the victim intended — a task often complicated by a victim’s failing mind, said Andrew Mayoras, a Michigan lawyer and co-author of “Trial & Heirs: Famous Fortune Fights!,” due out next month.

The question becomes: “How do you get into somebody’s head — someone with dementia?” he said.

Manhattan prosecutors called more than 70 witnesses to illuminate Astor’s mental state in the years leading up to her death in 2007 at age 105.

Jurors heard that the Alzheimer’s disease-stricken socialite signed a letter giving her son $5 million on a day she didn’t know where she was. That she was afraid of her shadow and didn’t recognize people she had known for decades. That she referred to her son, Anthony Marshall, at times as her husband and “the man who wants to kill me.”

The Manhattan District Attorney’s office, which declined to comment on the case, took some criticism during the trial for the lengthy presentation. But the testimony on Astor’s decline, especially from her doctors, made an impression on juror Barbara Tomanelli.

“We started to feel very sorry for Mrs. Astor’s condition,” she said after the trial.

Defense lawyers, who plan to appeal, also strove to peer into Astor’s brain, highlighting some of her letters and remarks to argue that she was lucid at points — including when she signed changes to her will that gave her son control of millions that had been destined for her favorite charities.

The defense also emphasized that Marshall had legal authority to give himself gifts as he managed his mother’s money.

“It appears that the jury may have held Tony to a stricter standard of spending than his mother did,” defense lawyer John Cuti said.

Marshall and lawyer Francis X. Morrissey Jr. are due to be sentenced in December; Marshall faces at least a year in prison and up to 25 years for 14 counts including grand larceny.