how was i supposed to know
open your profile in an incognito window
it's also worth separating "is under 18" from "signed out" and "doesn't want to see this NSFW post", those are all orthogonal. for a signed-in user, if they're under 18, they'll just turn off NSFW posts entirely; if they haven't, they'll probably look at the tags before opening the post, since they usually act like "light" CWs.
being signed out means you don't get to know anything about their age, so instead you get a UI that tries to capture both of those at once. you could argue that maybe there should be 2 click throughs, one based on age and the second being the normal "here's the tags and CWs, do you want to view this post", but that would make it more different to the normal website and be annoying for the users who do actually want to see NSFW posts signed out, which i would guess is most of them.
it's also worth mentioning that you can make your page entirely unavailable to signed-out users, no posts and (i think) no bio, which is probably better for a primarily NSFW account, instead of trying to half-sanitize things so the fewer SFW posts are still available to a small number of users.