The age of the earth is known to scientists for a very good reasion its called radioactive decay. All radioactive elements go through well known and measured "half-lives". This is the amount of time it takes for the element to release enough radiation for half of its material to become a more stable element, like Uranium-238 (the most common isotope of uraniam) which has a half-life of 4.468 billion years. Samples of uranium are more than half decayed meaning that the earth is older than the half life. Half life even very large ones can be measured by seeing the time it takes for a minute amount to deteriorate. Anyway the age of the earth shouldn't have to do with belief it's observation and science. Like evolution without which most modern medecine wouldn't make sense. Anyone who denies evolution should only be treateed with penicillan after all bacteria can't evolve resistance can they.