I started my eldest when he was about 24 months but it's always good to start earlier. I use spiffies for my second who is now 10 mths old.
I know it's diffcult for them to start to want to brush their teeth a that young age.
I started by asking my eldest to join me when I brushed my teeth. I tell him that's what big people do with their teeth, to keep them clean, the same reason as to why we bathe. If not germs germs will grow.
Then I show him library books on brushing teeth.
Lastly, I brought him to the supermarket to pick his favourite toothpaste and toothbrush.
Just my 2-cents contribution!