Despite their statements about the limited inferences that can be made without knowledge of the prevalence of young children sleeping in adult beds, Nakamura et al1 conclude that children younger than 2 years should not sleep in adult beds. Their conclusion is not supported by their data and bears on the question of whether co-sleeping causes increased risk of sudden infant death syndrome (SIDS).
Rosenberg KD. Sudden Infant Death Syndrome and Co-sleeping. Arch Pediatr Adolesc Med. 2000;154(5):529-530. doi:10.1001/archpedi.154.5.529