The disciples didn't specify the age at which they wished to die. Rather, they requested to live unto the age of man. The "age of man" was set by the Lord at seventy-two. Currently, the life expectancy for a man in the United States is 73.6 years, for a woman is 79.4 years. (1997 National Vital Statistics Reports, vol. 47, no. 28) Interestingly, the miracle of modern medicine has made the ideal "age of man" attainable for most.
Experts in human longevity and life expectancy have found that there are internal determinants of life expectancy. The rate at which DNA errors occur during cell division is fairly constant and has implications for the breakdown of cells and the development of cancer. Thus, even with the best set of genes, the best diet, and the best medicine, life cannot be extended much beyond the age of 100. The human body is just not designed to last much longer. One could argue that the Lord, as the premier scientist and physician, was well aware of the average rate of physical decay and therefore set the "age of man" appropriately.
For those who are alive when the Lord comes again, this age of seventy-two will again be significant. The righteous will not become glorified and resurrected beings immediately. Rather, they will grow to the age of seventy-two before this transformation will take place, he that liveth when the Lord shall come, and hath kept the faith, blessed is he; nevertheless, it is appointed to him to die at the age of man, Wherefore, children shall grow up until they become old; old men shall die; but they shall not sleep in the dust, but they shall be changed in the twinkling of an eye (DC 63:50-51, italics added).