How old is Mercury?

Short answer: About 4.6 billion years old.

Long answer: Everything in the Solar System was formed together at the same time, from a spinning cloud of dust and gas1. Our best estimate is that this happened 4.6 billion years ago. This means that all the planets in the Solar System are roughly that old.

Mercury was discover by humans in prehistoric times, so we don't know how long it's been known about.