We started at 12 months. In terms of follow-milk, my HV said they are not necessary if LO is having a healthy, varied diet. She said that one of the reasons formula companies make them is because they cannot advertise products to children under 6 months. By making follow-on milk, they can advertise the brand and (indirectly) the fact that it's not the first milk available.
ETA: I'm not criticising FF, just repeating what my HV with DS1 said about follow on milk.