My DS didn't get his first teeth until.... 10-11 months old maybe?? So he was already eating some solids and stuff, plus I wanted him to get used to toothbrushing BEFORE the teeth came in because I am a stickler for dental hygeine! So as soon as I saw those first 2 teeth about to poke through, we started brushing. We used a finger brush at first (still use it sometimes when he's teething) but also let him play with a regular (soft toddler) toothbrush and sometimes brushed with that instead, now it's almost always the regular brush. He's 16 mos & still only has 6 teeth!

We just use water, one time I tried using a bit of my homemade toothpaste but he did NOT like it

which is funny because he will eat anything...
We brush morning & night.... but we don't force it if he's fighting it (usually he likes it) -- we want him to have a good association with toothbrushing even if it means sometimes we only brush 'em for 3 seconds... luckily his teeth are in great shape, but I have really good teeth (never had cavity) so hoping he inherited them!
