That looks like a regular mature hairline to me, and the thinness at the top is just the light hitting your scalp with your hair in that whorl/spiral pattern.
I am pretty sure you are not balding right now.
What you'll want to keep an eye out for is (1) obviously any actual thinning of the vertex (you'll be able to tell the density is way lower if it starts to thin, don't go by the optical illusion of light hitting your scalp) and (2) any miniaturization of the hairs on the hairline. They'll start getting smaller and looking weak and thin...if you have any baby hair on your hairline (you might not), you can usually tell a difference between that and the miniaturized hairs which tend to be more brittle/hard/weird looking.
Otherwise, don't be worried about the mature hairline. Looks fine and doesn't look like its thinning at all to me.
I know what you mean though about other people with NW1 or NW0, I don't even know where I was at age 18 because I've always covered up my hairline a bit even when my hair was short, but only one guy do I ever remember in high school having a receding hairline (he was more or less thick NW3 but with an afro LOL)