well technically, you do start dying of "old age", which unfortunately comes at the ripe age of 30, varying for ages for different people of gender, race, size, composure etc. as someone stated above it is around the 30 year old mark that the body does not produce as many of the essentials we need to grow and to repair our body to the full, minerals, amino acids etc. We actually start to "die" faster than the body can repair, however slow the process takes, 50, 60, 70 years etc. And it is because of this that we become more prone to diseases, cancer, organ failure etc. When u say it is not "old age" that kills you it predominantly is, because we cannot fight off, and/or repair the body quickly enough to stop them from spreading etc. Have you ever seen a ripe teenager die from a cold? Very very unlikely, because the amount of white blood cells produced and efficiency of them is second to none. However it is not unlikely for a cold to progress and sometimes, tragically, kill an old person, because their body cannot fight the infection quickly and effectively enough. finishing... in a way it is "old age" that can lead to the death of someone.