Why date some one way to old for u

i had a friend who said he had an eighteen yearold gf
i was stupid and believed him(he was ten) I dont understand it

When a story sounds unbelievable it's usually because it's not true.

The years between ten and eighteen are the most critical period of your life. Any young person that tries to jump past them is making a big mistake. He needs that time for his mind to catch up with the changes taking place in his body. That requires experience, not just talk. If you can make your friend understand that, you will have done a good thing.