Astronomers estimate that the Big Bang occurred between 10 and 20 billion years ago. To put this in perspective, the Solar System is thought to be 4.5 billion years old and humans have existed as a species for a few million years.
Astronomers can estimate the age of the universe by studying globular clusters.
Globular Clusters are a dense collection of roughly a million stars. Stellar densities near the center of the Globular Cluster are enormous. If we lived near the center of a Globular Cluster, then there would be several hundred thousand stars closer to us than Alpha Centauri, our nearest stellar neighbor. The lifecycle of a star depends upon its mass . High mass stars are much brighter than low mass stars, thus they rapidly burn through their supply of hydrogen fuel.