Your question is very much a matter of opinion, but in order to have an opinion you must first understand what the speaker of the poem is saying. Sonnet 12 is about the inevitable end of all living things. He references the end of day into "hideous night, a flower "past prime" and faded, once dark hair that has turned white with age, bare trees, and absent birds. All of these images of the opening octave lead to his conclusion that all everything "sweet and beautful" must fade and be subject to "Time's scythe." A scythe is usually associated with the harvesting of crops -- of in general life. The speakers ultimate point comes in the final couplet. To paraphrase he says that nothing will work against Time's cutting you down except the fact that you can "breed" or have children. By having children you are, in a small sense, living on and therefore you are defying death.
Is the passage of time inevitable for everyone? Yes. But the question really becomes how are you going to make the most of time you have. One answer is to have children. But other answers are as varied as the people you would ask. A person could defy time by doing things that will have a lasting impact on the world -- big or small: helping the needy, fighting in a war, teaching others, researching in fields related to improving our way of life, etc. Many times we live on through our actions in ways that we never know.