Once their first tooth starts to poke through
With my first born we used to finger toothbrush just to gently brush the tongue, you don’t need to touch the gums because the main thing to worry about is the oral thrush build up on babies tongue that’s the last thing you want & I highly disagree giving any child adult toothpaste especially as a baby, even when their teeth come in they’re nowhere near strong enough to handle the fluoride level in adult toothpaste
Dentists recommend normal adult toothpaste, didn’t start until had teeth