As soon as we started brushing with a toothbrush, we used the infant toothpaste, then moved up to the toddler toothpaste after that. It has stuff that is different than an adult toothpaste, since most can't spit at that young of an age.
We started brushing we he got his first tooth, but didn't use toothpaste until he really, really fought getting his teeth brushed. He was probably around 16 months when we started using the Oral-B without fluoride.
Re: speaking of teeth brushing...