My son is 16 months old and got teeth pretty late, so he's only got 6 right now and 4 aren't even all the way through. We just started brushing teeth, but he won't let me do it. So I just give it to him and model how to brush. When I brush my teeth in front of him, he gets a better idea of what to do, and will brush a bit. I'm sure it's not actually getting his teeth *really* clean, but we do this twice a day, and I think it's great for his age.
Once he has more teeth and is older and eating more sweets (right now the only sugar in his diet is the occasional Popsicle) I might push the brushing a little more, but for now, I'm completely satisfied with letting him enjoy tooth brushing and doing it himself. I really feel like if I forced the issue, it would end up a battle that would make him hate brushing his teeth, and I'd rather have it be fun right now.
If your able to keep it fun and still keep them really clean, then that is great, just keep an eye on things in there