I didn't really floss until a few years ago. Every time I went to the dentist, she would remind me to floss. I knew it was important, but I still didn't do it. It wasn't a habit.
Then one dentist appointment, as she was checking my gums, she said that gums are like your GPA: you start with a 4.0, and if you don't maintain it by flossing every day, your GPA will drop, meaning your gums would recede and you will never have a 4.0 again.
That analogy scared the hell out of me, and I started flossing that night, and I've been doing that every day since.
I think it was the thought of permanently and irreversibly damaging my body that scared me. But that experience also reminded me that it's never too late to start. Ever. You're gonna keep on living, and since you have to floss every day, so today is the best day to start. Even that night, it felt like it was already too late, I already messed up, but looking back now after a couple years, I'm glad I started.
James Clear says one way to start is to just floss one tooth. Just one. Make it impossibly easy to start. That you're feel like an idiot if you can't do it. Because even an idiot can floss one tooth, right? And then floss two teeth the next day, and practice so the motion becomes a habit!