The atoms that make up your body are indeed very old, with many of them having existed since or shortly after the Big Bang. However, it is important to understand that the atoms in your body are not fixed entities. They can be recycled and exchanged with the environment through various processes.
The elements that compose your body, such as hydrogen, carbon, oxygen, and so on, were primarily formed through stellar nucleosynthesis. In the early universe, after the Big Bang, the only elements present were hydrogen, some helium, and trace amounts of lithium. Heavier elements like carbon, oxygen, and nitrogen were synthesized within stars through nuclear fusion processes.
As stars go through their life cycles, they eventually exhaust their nuclear fuel and undergo stellar evolution. During this process, stars can release their enriched material back into the universe through stellar explosions such as supernovae or stellar winds. These expelled materials, which contain heavier elements, then mix with the interstellar medium.
Subsequently, new stars, planetary systems, and even life can form from these enriched materials. The process continues as stars are born, live, and eventually disperse their elements back into space. Therefore, the atoms that make up your body could have been part of several generations of stars before becoming incorporated into your being.
In terms of the growth of your body, the additional atoms needed come from the food you consume and the air you breathe. When you eat food, your body breaks down the molecules and extracts the necessary elements to build and maintain its various structures. Similarly, when you breathe, you inhale gases like oxygen and exhale carbon dioxide, exchanging atoms with the environment.
So, while the fundamental building blocks of your body's atoms may have existed since the early universe, the specific atoms that currently comprise your body are constantly being exchanged with the environment through metabolic processes, food consumption, and respiration.