Can a 18 year old date someone older?

People, in most states, who are 18 or older are considered adults. It is not illegal for an adult to date another adult. Further, even if one of the people is under the age of 18 they can date as long as their parents consent to the dating.

Can an 18 year old date a 25 year old man?

No it is not against the law. May not be a wise idea, but it is legal.

What is the oldest a 18 year old can date?

If you are 18, at least in the US, you can date anyone you want who is also over 18 and consenting. As for what is recommended, I believe the general rule is “half your age plus 7,” so if you are 30 years old, the youngest partner you should have would be 22 years old.