Before 1970, no mixed-age groups existed in Swedish nursery schools. By 1991, 43 percent of children enrolled in nursery school were in mixed-age groups of ages 1 to 6 years, and 37 percent were in groups of children ages 3 to 6 years. Mixed-age groups are assumed to have advantages, including positive influences on learning and social development, as well as increased teacher satisfaction. However, a review of comparative studies investigating the effects of age span on children's learning and socio-emotional development and on teachers' satisfaction reveals conflicting conclusions. To determine the effects of mixed-age instruction, additional evaluative research is needed. (MM)