As inmates grow older and more elderly convicts join their ranks, there is a growing senior prison population --costing state systems dearly.

''Crime is a young man's game,'' declares Wilbert Rideau, 55, who has spent the past 37 years in the Louisiana State Penitentiary on a life sentence for murder. But with longer sentences and stricter parole policies, ''prisoners are getting older.'' In 1996, he says, for the second year, more inmates of his large prison died than were paroled. Louisiana is hardly alone. In California, the number of inmates over 50 increased last year to 6,735, or 4.8 percent of the total, compared with 3,874, or 4.0 percent, in 1990. In Texas, with another big prison system, 9,130 inmates, or 6.7 percent of the total, are currently over 50, up from 4,212, or 6.1 percent, only three years ago. By one estimate, now considered conservative, there are 55,000 inmates over 50 nationwide, a figure that could reach 125,000 by 2000. While the numbers look small for now, prison officials are alarmed, for the growth has begun well before the full impact of ''three strikes'' and other mandatory sentencing laws.

That's an issue for state taxpayers as well as prisoners. Inmates aren't eligible for Medicare or Medicaid, and age leads to illness, more behind

bars than in the free world, given the routine of chronic stress, cigarettes, idleness and heavy food. Federal judges have ruled that failure to provide adequate medical care violates the Eighth Amendment's ban on cruel and unusual punishment. As a result, prison systems wind up paying the full cost of treatments for cancer, heart disease, prostate problems, emphysema, strokes and Alzheimer's. Researchers find that an inmate over 55 costs three times more to incarcerate than the average inmate.

Some prisons respond by trying to contain health costs. Texas recently shifted medical services for its 135,000 inmates to a managed care program run by state medical schools; it saved $38.6 million in fiscal 1996. Prisons are also expanding units that provide long-term nursing for the frail and hospice care for the dying.

Another approach, for humanitarian as well as financial reasons, is to identify and parole aging prisoners whose crimes are long forgotten and who are unlikely to commit new ones. Jonathan Turley, a George Washington University law professor, mobilizes students to represent aging inmates. So far, they have managed to free 168, and none have been rearrested. The process is complicated, for though most aging inmates are long-termers, they are sometimes joined by what Turley calls the ''late bloomers and overachievers.'' Alabama prison officials recently had to deal with an 81-year-old man sentenced to five years for theft of property. (He took money to put up siding, then failed to do the work.) And a 72-year-old is serving a life sentence for shooting his girlfriend.

The special housing and release programs fall far short of the need. Many aging inmates find themselves struggling to cope as waning physical strength and mental alertness make them targets for the younger prisoners who fill most of the beds in medium-and maximum-security cellblocks. Rideau, who edits The Angolite, a prison magazine, is also president of an inmate club that recently decided to ''adopt'' all prisoners over the age of 60. ''We wanted to serve notice that they are no longer alone, in case anybody might be thinking of scamming an old man out of something, jacking him around.''

The greater threat, beyond illness or victimization, is despair. Many prisoners, Rideau observes, ''put their lives on hold.'' As they age, they begin to face the fact that they may never get out. ''The dream of getting out, you equate with heaven. Dying in prison, you equate with hell.''

Photos: Inmates at the Hamilton, Ala., prison for the aged and infirmed, known as A&I, gather at an outdoor area designated for smoking. Opened in 1981, the prison can hold 250 convicts. (pg. 28&29); Jesse Rodriguez Trevino, 62, has been in and out of Texas prisons since 1957. In 1988, he began serving his most recent sentence -- 25 years with no parole for assault with a deadly weapon -- in a geriatric ward in a Huntsville, Tex., prison. Adam Cook, 72, arrived at A&I in 1994 with a life sentence for shooting his girlfriend to death. Since then he has lost both legs to diabetes. Donald Shepard, 30, who is changing Cook, is also an inmate, serving 18 years for theft. (pg. 30); Despite heart problems, Truman Purdy, 61, lifts weights in the Hamilton, Ala., prison exercise yard. His 25-year sentence for sexual abuse and sodomy began in 1992. Ailing inmates gather in A&I's common room. Doctors generally make the decision to send inmates to A&I if because of advanced age, injury or illness they cannot function in the state's regular prisons. (pg. 31); Riley collings, 54, wheels himself through the yard at A&I. Convicted of sexual abuse in 1996, he was sentenced to five years. (Photographs by Ed Kashi)(pg. 32)