How old is Mars?

Short answer: About 4.6 billion years old.

Long answer: The Sun, the planets and almost everything else in our Solar System were all formed together from a spinning cloud of dust and gas1. Our current best estimate is that this happened around 4.6 billion years ago, which means that Mars is that old.

In other words, with a few exceptions such as moons and asteroids that may have formed more slowly, everything in our Solar System is the same age: 4.6 billion years old.