I am 18. My hairline dips down in the middle and then becomes much higher at the sides. Is it receding or just naturally curved?

There is no good way to tell what is going on without an examination and detailed history.  It can be anything from genetic to some unknown cause.

From what you have described it sounds like this is a familial (genetic) pattern.Certain preventative measures may be appropriate if hair loss is suspected.18 is a bit young for transplants but if it concerns you schedule an evaluation with a Dermatologist or experienced Hair Restoration surgeon.

It could be hereditary. I suggest seeing a hair loss expert to evaluate and go over your condition and provide you information about your best treatment options. There are great non-invasive options like PRP/progesterone and/or hair transplantation. See an expert. Best, Dr. Emer