Hmm, I would guess it's not thinning. If that's how your hair looks after a shower, it's very strong and thick. I'd say give it 6 months, see if it gets worse. If not, it's probably just a parting. Your hair looks really good and strong. No worries.
If it does get worse, Finasteride can be cheap and easy to use. You may have a tough time getting it as an 18-year-old, but it'd be worth a try if that time comes. But I really don't think that's necessary.