Geriatric comes from Greek geron, 'old man', and iatreia, 'the treatment of disease'. It is used medically, simply to describe the treatment of elderly patients, or in a slightly insultory fashion, referring to something old and decrepit. Indeed, being old usually leads to deterioration in a lot of things. Elderly people may suffer from loss of agility, memory, and senses, in addition to having one or more of their organs acting up. Mainly this is because their cells are getting old. Or, more accurately, their DNA.
Let's go down to the cell level. A normally functioning, specialised cell is programmed to cut a piece of telomere off the tips of its chromosomes with each cell division. Therefore, the more times the cell has been divided, the less telomere it has. When there is no more telomere, the cell reaches a resting state called senescence, and no longer divides. This is why old people take much more time to heal than young - they have less cells that are able regrow the injured tissue. Eventually an organ will contain so many senescent cells that it is unable to renew itself, and it will fail.
Another way in which DNA is altered is through faulty copying. Mistakes happen, and are not always corrected. Eventually a cell may mutate so that it grows uncontrollably and does not die the programmed
cell death. This cell divides into a colony of cancerous cells and wreak havoc on the whole system. The older a person becomes, the more likely he is to have produced all the necessary mutations for some cell to run wild and give him some form of cancer.
Humanity is, as always, looking for ways to live forever. Old people die, so how can we stop the aging? Current research aims at inserting new cells into old organs. With all the telomers in place, the organs might be able to function properly for another lifetime. As for a cure for cancer, science fiction ideas suggest somehow repairing the DNA in all the cells. Bingo, no more mutations and no more cancer.
But for the moment, we still have geriatrics, both old worn-out people and doctors who treat them. Which one is the disease, old age or life itself? As far as we know, no one has been able to survive it so far.