Late school entry is driven by several factors, one of the key ones being the cost barrier to schooling. Policies such as free primary education (FPE) that advocate for universal coverage are therefore partly aimed at removing the cost barrier. The Kenyan Government, like many in sub-Saharan Africa (SSA), introduced FPE in 2003 with the aim of universalising access to schooling, which is one of the eight United Nations Millennium Development Goals (MDGs) it signed up to achieve. Based on a case study of four sites in Nairobi, the aim of this paper is to assess whether the FPE policy has affected late enrolment. The data used were collected by the African Population and Health Research Center (APHRC) and comprise a sub-sample of 4,325 first-graders during 2000-2005. The paper applies a probit model to assess the impact of the policy on the basis of marginal effects on the predicted probability of late enrolment. The results show that the FPE policy reduces the probability of late enrolment by 14 per cent. The reduction in probability of late enrolment was greater among children residing in slums (16 per cent) than those in non-slums (9 per cent). The main implication of the findings for policy makers is that cost barriers are a likely cause of over-age enrolment.