If you’re like me, then some time in your thirties. I didn’t brush from early teens through until then - I had several abcesses and needed seven teeth removed, including my top fronts. Turns out I had undiagnosed autism, depression and low self image. Now I do brush, and it’s just a case of forcing myself to adapt to a routine. Even keeping some flouride mouthwash handy for a quick swill every now and then helps a bit. Hope you find your way.