Calves can develop long-lasting social relationships with peers. We examined the strength of the relationships between calves according to the time they had been together. Twenty-four female dairy calves were assigned to six groups of four animals (Type-1 partners) at 0.5 month of age. At 3.5 months of age, they were mixed with other calves (Type-2 partners) to form groups of 14. Type-3 partners were calves added to the experimental groups after 5.25 months. The calves stayed together until 1.5 years of age. Social preferences between the three partner types were examined in a Y-maze, and the position and activity of animals in the barn and pasture were followed in three periods. Behavioural synchrony, distance between animals, proximity and nearest neighbour were analysed. The calves more frequently butted Type-3 than Type-1 partners in the Y-maze (P < 0.05). They spent more time in proximity to Type-1 partners, and these were more often the nearest neighbours than other partners (P < 0.001). Synchrony and distance between animals were greater at pasture than in the barn (P < 0.01). Calves seem to form preferential relationships before 3.5 months of age. Keeping cattle together from an early age seems beneficial for them.