Final answer:
We know Mercury's surface is much older than Venus's because Mercury is heavily cratered, indicating an old surface. Venus has a younger surface age due to geological activity that resurfaces the planet. The lack of recent geological processes on Mercury preserves its ancient craters.
Step-by-step explanation:
The surface of Mercury is much older than that of Venus primarily because Mercury's surface is heavily cratered compared to Venus. This high density of craters on Mercury's surface is indicative of its age since the number of craters on a planetary surface correlates with how long that surface has been exposed to space, assuming a relatively constant rate of impact over time. Venus, by contrast, has a younger surface age, estimated at only between 300 and 600 million years, due to its geological activity that resurfaces the planet and removes evidence of older craters.
There is also no evidence of plate tectonics on Mercury, which would otherwise contribute to the recycling of the planetary surface. The planet's distinctive long scarps and the absence of plate tectonics imply that its surface has not been renewed in a significant way since they were formed, and these features must have developed after most of the craters already existed. Unlike Venus, with persistent geological activity, Mercury lacks such processes, making its cratered surface a witness to events that happened billions of years ago.