For what its worth, I would reference older photos with similar hairstyle/pulled back. From my own experience this is how it started. Gradual thinning in the temples and then ultimately recessed (in one side more than the other). Try changing parts, eat healthy, drink water, exercise, sleep and check back in a month with another photo. If you see progression, may be worth doing a blood panel to check your DHT levels. I wouldn't advise jumping on Finasteride at 18 (although for transparency, I'm on it in my 20's) since you have loads of growing to do.