The reason we observe stars that are billions of years old but not stars that are trillions of years old is related to the age of the universe and the process of stellar evolution.
The age of the universe is currently estimated to be around 13.8 billion years. According to current scientific understanding, the first generation of stars began to form a few hundred million years after the Big Bang. These early stars, known as Population III stars, were composed almost entirely of hydrogen and helium, as heavier elements had not yet been synthesized through stellar nucleosynthesis.
Population III stars were short-lived and massive, with lifetimes on the order of a few million years. They eventually exploded as supernovae, dispersing their enriched material into the surrounding space. The remnants of these early stars and their subsequent stellar explosions seeded the universe with heavier elements.
Over time, subsequent generations of stars, known as Population II and Population I stars, formed from the enriched gas and dust. These stars contain a mix of elements beyond hydrogen and helium, including heavier elements like carbon, oxygen, and iron. Our Sun is a Population I star.
The reason we don't observe stars that are trillions of years old is that the age of the universe is not old enough to support their existence. The oldest stars we observe are typically around 13 billion years old, close to the age of the universe itself. Trillions of years would greatly exceed the current age of the universe, so we haven't had enough time for stars to evolve and exist for such extended periods.
It's important to note that our understanding of the universe is based on scientific theories and observations, and as our knowledge grows, our understanding may evolve, leading to new insights about the early universe and the formation of stars.