If the world is millions of years old, why is it only 2018?

Because it has only been 2018 years after it switched to A. D. it was B. C. before that.

B. C. stands for before Christ (Jesus) A. D. stands for after died

Well, first we had B. C. E which was before Jesus was borned and this era lasted a LOT. Then, came C. E which is now our Era or how people call it: The Tech Era. (P. S B. C. E Means Before Christ's Era and C. E Means Christ's Era or Common Era.)