Question

how fossils are formed? how we can determine the age of fossil ?

Answer 1

Scientists combine several well-tested techniques to find out the ages of fossils. The most important are Relative Dating, in which fossils and layers of rock are placed in order from older to younger, and Radiometric Dating, which allows the actual ages of certain types of rock to be calculated.

Answer 2

Fossils are formed when some parts of organism don't get decomposed or when their impression is formed on hot mud which later sets.
We can determine their age by carbon-dating and by the position of fossil in the layers of earth.