Supposed to be 12mths, but you might want to introduce a little drop now and then to get her/him used to the taste. Some people use formula beyond the first year as it contains vitamin and mineral supplements, but this is probably unnecessary if your LO has a varied diet.
Am not an expert (as mine are BF) but I used cow's milk in their food from 6mths and tried to give them some from 12mths. DS1 loved cow's milk and drinks it by the pint even now, DS2 is less enthralled. But I've also read that as long as they are getting lots of milk-products in their diet (e.g. cheese, yoghurt) and milk in cooking (e.g. in porridge), then that counts towards the pint a day that they are supposed to be getting at around this age.