How do you prove that Earth is older than 10,000 years?
Any adult with a cellphone, who still believes the earth is literally less than 10000yo is a lost cause.  Give up.  Focus on making sure that they dont infect future generations.  Make sure the grandkids know that grandpa says strange things sometimes.  One cannot argue reason to a true believer.  It only makes them stronger.<p>Or buy them a seat on that flat-earth rocket.