If the baby is bottle fed. it is vital that the teat respects physiology, that is the naturalness in sucking.
It must be then extremely soft, elastic, extensible, so to not hinder any movement of the mouth or the tongue of the baby during the nursing, in the same way of breast feeding. But not only that, a good teat must respect the the way the baby sucks and swallows which change and evolve over the months, as proven by scientific evidence in this field.
In fact, in babies the swallowing mechanism is a constantly evolving dynamic process: it was proven actually that, from birth up to 6 months, during the nursing the baby keeps his mouth wide open, the lips wrap around the nipple (or the teat) and the baby opens and closes his mouth, and the tongue slides back and forth to move the milk towards the back of the mouth so it can be swallowed.
At this age, it is advisable to employ a very extensible teat, whose shape is broad and rounded, so that the baby's mouth is wide open. After 6 months, however, the baby’s lips are more closed and the tongue moves so to create a wave that helps suction (aspiration) and the swallowing of milk. It is then ideal a more elongated teat, that allows a narrower latching of the lips.
So, for following the evolution of the mode the baby eats, without interfering the suction physiology, the teat must have a shape, dimensions and features which are different for each single age range.