Neurotypical here, as far as I know.
No, there is nothing about brushing my teeth that's automatic. At some point during the evening I usually notice fuzzy-feeling teeth, or as I'm nodding off while reading my book will think "oh no, I never brushed my teeth..." There is always some small element of discipline to getting up and doing the thing. But I can usually remember without having to set an alarm or post a sticky note, if that meets your definition of "habit".
Exercise is brutal and IMO, that never gets easier, either. In contrast, I generally have an easier time with mental discipline & focus compared to physical tasks.
Strangely enough, as a (mostly) white Millennial woman, the majority of my peers now claim to have adult-onset/adult-diagnosed ADHD. Maybe this is an accurate diagnosis for some. And even if it's an exaggeration in other cases, who cares, as long as the coping strategies or medication is improving someone's quality of life.
What's sad to me, though, is when the diagnosis becomes an all-consuming identity and an excuse to stop trying altogether, a way to shut out the rest of the world. These women I know who excelled in school and work or had creative hobbies and traveled the world, now they just post mental health memes all day and joke about how it's impossible to get out of bed. And if anyone suggests maybe they TRY getting out of bed and see how it feels to participate in X, Y, Z activity like we used to, then come the accusations that we're clueless NTs who will never understand what it's like to struggle.
And that's just not true. NTs also work hard and struggle at times...that's life. So let's just make sure all these diagnoses are helping and working FOR us rather than AGAINST us. And sometimes forgetting to brush your teeth is just ... forgetting to brush your teeth.
-Here's the weirdest one: I used to hate the French language (too "mushy"!) and was super excited that my college offered German. I took THREE YEARS of German. Now I've forgotten it all, but I'm really into French and actually considering a French immersion program to get more conversational.
-Also, I used to love loud & stimulating environments (metal shows, night clubs, etc.) while now I appreciate tranquility. Gimme a used bookstore or nature walk any day! I went to a drag brunch recently cause it seemed like the kind of thing I should like as a gay-ish Millennial woman. But it was SO not my thing ... the lights, blaring music, close bodies, and cell phones documenting every second just ruined all the nice/chill things about brunch. And I can't survive an actual concert without earplugs.
-Food-wise, I hated seafood as a kid. Now I love it, including oysters and scallops and octopus and sushi/ceviche of any kind.