Sixty low-income adolescent mothers of children ranging in age from birth to 28 months were asked to estimate the ages at which children are first able to engage in specific activities in cognitive, language, motor, play, and social development. In general, mothers were quite knowledgeable about the ordering of developmental abilities relative to one another, but were less knowledgeable about the developmental timing of abilities. With respect to timing, mothers tended to underestimate the onset of all developmental abilities. That is, mothers predicted that children's developmental abilities would appear earlier than they actually do. Mothers' knowledge about cognitive, language, and motor abilities was stronger than their knowledge about abilities in play and social development. Across domains, mothers were more accurate at estimating the ages of abilities that emerge in the first year than those occurring during children's second and third years. Findings are discussed with respect to societal emphases on different domains of development, and the potential implications of unrealistic expectations for adolescent parenting. ©2002 Michigan Association for Infant Mental Health.