You seem to be using evidence to back up your claim that Mt Everest is not more than 6000 years old. Ok

Scientists (geologists/geochronologists) also use evidence to back up their claim that Mt Everest is very old. Their "educated guess" uses a lot more education/knowledge than guessing. You seem to be saying that they just made it up with no evidence to support it at all.

An analogy:

You are trying to determine a person's age. That person has white hair that is quite sparse on the top. There are deep deep wrinkles on his forehead and around his eyes. His knuckles are swollen and misshapen. He even has wrinkles on his knees. His posture is very stooped and he uses a walker to slowly get around. When he talks to you, he tells stories about being a kid in Chicago when Al Capone was running the city.

Would you guess that he was 6 months old? Of course not, because you see the indicators of age and understand how long it takes for those indicators to show up on a person.

There are a number of issues with this paragraph. You seem to believe that the Earth is static even though it has been pointed out to you that the Earth's continents are moving. Mt. Everest is actually rising due to the collision between the Asian continent and the India subcontinent. The rise is very slow, on the order of centimeters, but it is faster than erosion in that area. Some places in the world are rising and making mountains while other parts are eroding away.

We don't have all Grand Canyons because, besides being logically impossible, erosion rates are different in different parts of the world. The main cause of the Grand Canyon's erosion is the Colorado river.

Scientists see the wrinkles (Grand Canyons) on the Earth. They see the massive accumulations of sand in sandstone and in loose piles. They even see rocks that are pieces of sand that have hardened into sandstone then eroded away from the sandstone. They see continents that used to be touching each other are many miles apart.

The scientists have investigated how fast things erode, and how fast the continents are moving apart. Thus they have an "educated guess" about how long it would take to make all that sand and dig the Grand Canyon. These scientists know enough about how geology works, that they can tell if something eroded away fast or slowly.

Based on their observation of the existing features of the Earth and their knowledge of the time it takes to get those features, scientists conclude the Earth is very old. Just as you would have determined that the person mentioned above is quite old.