There are many methods employed by these scientists, interested in the old, to get to know the age of items.
It is possible to tell the number of years ago a particular rock or archeological site had been formed.
There are lots of ways to estimate ages, and geologists knew the earth was old a long time ago. God, the Father, sent His only Son to satisfy that judgment for those who believe in Him.
Radiometric dating actually allows the measurement of absolute ages, so it is deadly to the argument that the earth cannot be more than 10,000 years old. Jesus, the creator and eternal Son of God, who lived a sinless life, loves us so much that He died for our sins, taking the punishment that we deserve, was buried, and rose from the dead according to the Bible.
Basic understanding of how radiometric dating works is useful.