This sounds like "The Time Beyond Age" from the short story collection The Bone Forest, by Robert Holdstock.
The day before the experiment was scheduled to commence, Martin and Yvonne, our two MAA-grown subjects, were allowed into the observation laboratory for the last time.
The first stages, of course, were the familiarization procedures, and our two subjects were introduced to the closed environment that was to be their home for the rest of their natural and unnatural lives. There was something almost depressing in watching the children, conceived, grown and matured to the age of six in a Morris Artificial Amnion, now facing an incarceration in a second womb, this time for good.*
A breakthrough into what was difficult to say. The kudos for the original discovery of Chronon was not ours. The chemical of age, the simple protein that accumulates in body cells at a steady rate and dictates the phenomenon of aging, was the discovery of a Swedish biochemist seven years before. The acceleration of development, and of aging, under the influence of artificially high concentrations of synthetic Chronon, was the contribution of a Scottish behaviorist at the University of Edinburgh, four years later.
The excitement of year one hundred passed into our distant past, and over the course of weeks, then months, the enormous ages reached by the subjects failed to arouse even the slightest whimper of joy. We worked virtually full-time countering the efforts of each of their bodies to shift into a disease condition.
One corner of the laboratory was filled with neatly stacked printouts representing the last fifty years of the subjects' lives, and staring at that pile of information I realized that nowhere in its bulk could I put my finger on a single statement of feeling, of awareness. Even the sheets on which were recorded the last living moments of Martin and Yvonne were bare, sterile accounts of failing physiology and murmurings and alpha waves; there would be no account of what they thought, what they felt as death unfurled its protective wings about them.
According to the Wikipedia article, it was originally published in "Supernova" in 1976.
From the Wikipedia article:
This is a science fiction short story describing a scientific experiment in which two subjects’ rate of aging is increased in an environment that is devoid of all disease. The subjects live for the equivalent of centuries and evolve into exaggerated archetypes of male and female. Eventually they die, but not before the perpetrator of the experiment self-administers the life-altering treatment.