Girl is 5yrs younger than boy

Whether she likes you or not, you are 18 and she is 13. Stay away. Stay far away. In the eyes of the law, you are an adult and she is a child.

Even if relationship isn't illegal in the your country, the age difference makes the relationship ethically and morally wrong. If it is in fact illegal in the your country, don't complain here when you're charged with statutory rape and thrown in jail for several years.

Also, even if she's considered mentally and/or emotionally older then her physical age, it's still morally and ethically wrong to pursue a relationship with her.

Yes, 13 is too young for you. In the US (and in many countries) you can go to jail for this relationship. A 13 year old is still a child, think about how much you are different now then when you were 13. Leave her alone.

You should find someone closer to your own age.

Having a relationship doesnt mean i m going to get married now only,what after 10yrs,i would be 28 n she 23,dats pretty normal then

The issue is that you are an adult and she is a kid. Adults aren't allowed to have sex (of any form) with kids. It is not only disgusting, it is illegal in most places.

If she were 23 years old, she would be an adult too. That case is completely different.

Due to your age, I think you should just tell her that you are not interested as she is too young for you. If you develop and begin a relationship with her, and you are sexually intimate with her, you could be criminally charged, or you will at least be known as a pedophile where you live.