Universe is around 13.8 billion years old. Most astronomers use globular clusters to calculate the age of the universe. It is the group of million stars. As all the globular clusters are made at the same time so they work as a cosmic clock in the universe. This is how we calculate the age of the universe.
To calculate the age of the universe, we need to understand the Big Bang Theory where the Hubble constant is a term used by astronomers to measure the age of the universe. The Hubble constant is used by astronomers to define the age of the universe. If we consider the universe as a flat object and made of normal matter then the age of the earth would be considered as 2/(3H0) but if we consider the universe as low dense matter it would be considered as 1/H0.
Most of the astronomers in the world are working to understand the exact age of the universe. A team of international scientists led by the Vendee Freedman used the Hubble telescope to see the real position of celestial objects and calculated the age of the universe as 9 to 11.5 billion years.