According to the Big Bang theory, the age of the universe is 13.8 billion years and this figure is accurate to within 100 million years. That means everything in the universe, as we know it, is at most that old.
Yet astronomers have found stars that appear to be 14.5 billion years old. The age of a star is determined by measurement of various factors, including its metallicity, i.e., the presence of elements other than hydrogen and helium in the star.
A Digitized Sky Survey image of the oldest star with a well-determined age in our galaxy. The ageing star, called HD 140283, lies over 190 light-years away. The Hubble Space Telescope was used to narrow the measurement uncertainty on the star's distance, and this helped to refine the calculation of a more precise age of 14.5 billion years (±800 million years).
It is possible that more accurate measurements will bring the numbers more in line in the future.