[Maru]

Doesn't the forehead hairline raise up before the rest starts to noticeably taper off? I'm sure that's not always the case, but I would also look at your hairline at your forehead...

My uncle lost a lot of his hair before 18, but I think that if that were the case, it'd be fairly noticeable by now...

Stress can also be a reason for a temporarily loss of hair in certain areas. Especially if you like to pull and tug at your hair a lot and scratch your scalp. You can try switching to a shampoo that helps treat the scalp, like Nioxin. It helped mine to thicken back up and regrow after having been on a lot of medications and having gone through a particularly stressful period...