The American Academy of Pediatrics suggests that bed-sharing between a child and parent at any age. The issue with infants (children under one year) is the number of children dying from SIDS, which is greatly reduced by “avoiding soft bedding, bed-sharing, and exposure to tobacco.”  Past the age of two years, the practice causes serious problems for the child, emerging at about age five or six.
“Bed-sharing at ages 1 to 3 years was bivariately associated with poorer behavior and cognition at age 5 years.”
This assertion is based on a study of 944 families who practiced bed-sharing to age five, with assessments done at ages 1, 2, 3, and 5. The study was conducted by R. Gabriela Barajas, MA; Anne Martin, PhD; Jeanne Brooks-Gunn, PhD; and Lauren Hale, PhD. The first three listed are professors at Teachers College, Columbia University, New York, New York; and Hale is a professor at State University of New York at Stony Brook, Stony Brook, New York. 
Another study examined the “duration of bed sharing has an impact on children” and the serious negative “effect [durative] bed sharing had on a child’s psychosexual development.”  This often includes premature fascination with genitalia or other erogenous zones (breasts, posterior, and mouth-to-mouth kissing) manifesting as early as three years of age.
Several states (including Michigan) define bed-sharing as an indicator of neglect, with Child Protective Services requiring that each child in the home has his or her own bed and clothing. 
 “Sudden Unexpected Infant Death Legislation,” National Conference of State Legislatures (March 2015).
 Lauren Hale, et al., “Mother-child bed-sharing in toddlerhood and cognitive and behavioral outcomes,” Pediatrics (doi: 10.1542/peds.2010-3300, published online 18 July 2011).
 S. Jain, R. Jain, & R. Romack, “Bed sharing in school-age children,” Journal of Child & Adolescent Psychiatric Nursing (2011 Aug) 24(3):185-9 (doi: 10.1111/j.1744-6171.2011.00293.x).
 Lisa Kirsch Satawa, “Protecting Your Family When CPS Comes Knocking” (22 Mar 2011; online: http://www.protectingyourfuture.info/protecting-your-family-when-cps-comes-knocking).