Is it OK for a 16 year old to date a 18 year old?

No this is not illegal. Simply dating a person over age 18 is not illegal. It can become illegal for a person who is 18 once sex is involved.

Is a 12 year old and a 16 year dating?

yes, it is fine for a 12 year old to date a 16 year old as long as they dont engage is any sexual activity until they are both at least 18 years old.

Are 11 and 13 year olds allowed to date?

No. It is not normal. Males should date someone at least 5 years younger than they and girls someone at least 5 years older due to both maturation differences and so that the girl will be treated with some more patience.