The Minions are as old as Earth
Many audience members believe Gru met a Minion in his early days and decided to make multiple copies of the first Minion to help him achieve his evil plan. However, it has since been confirmed that Minions existed long before human beings.
This can be seen in the short movie called Orientation Day, as it shows the idea of how the Minions came to be. If Minions were older than men, they would have probably existed during the era of dragons. Evolution made many Minion characters die off, and the existing character was cloned by Gru, creating more Minions.