This age is determined by:

* Dating the oldest stars: Scientists study the composition and age of the oldest stars in the Milky Way, which gives an approximate age for the galaxy's formation.

* Comparing to other galaxies: Studying other galaxies and their ages helps us understand the evolutionary history of the Milky Way and how old it is.

* Computer simulations: Models of galaxy formation based on our understanding of physics and cosmology provide estimates for the age of the Milky Way.

It's important to note that this is an estimate, and there is some margin of error. However, the current best estimate puts the age of the Milky Way at about 13.6 billion years old, making it nearly as old as the universe itself.