Found this via Best Evidence in Brief:
A new report summary from the Institute of Fiscal Studies finds that impact of early education fades as children go through school.
In England, all 4-year-olds have received free, part-time early education since 2000; all 3-year-olds have received it since 2005; and low-income 2-year-olds since 2013. However, the introduction of these free services was not immediate, and this has allowed the researchers to measure its impact on child development.
The researchers found that the introduction of free early education for 3-year-olds improved their outcomes slightly. Children’s development is assessed at age 5 using the Foundation Stage Profile, and average scores rose from 87.5 to 89.3 (out of a possible 117). These small impacts came mostly from those children who would not have attended early education without the free entitlement. If it is assumed that all of the increase comes from these children, then their scores would have risen almost 15 points on the Foundation Stage Profile.
The researchers followed the children to ages 7 and 11, when children take further national tests. The estimated impacts of the free education at age 7 were very small, and by age 11 they had disappeared entirely.
The policy of free early education was introduced because of the EPPE study, which showed that children who received preschool in the late 1990s started school with better cognitive development, and these effects persisted to age 11 and beyond.
The authors suggest a couple of reasons why their research finds differently. Free child classes are now often in private, voluntary and independent settings, which may be of poorer quality. Alternatively, primary schools have changed and improved since the late 1990s, and so preschool experience may now matter less.