To calculate the cohort admission rate you can choose a given cohort - e.g. that born 10 years ago - follow it for several consecutive years and account for the members of this cohort who successively sooner or later enter school. It is (also) possible, if one has for several years the age-specific admission rates, to reconstruct a cohort admission rate.
A more appropriate means of measuring admissions is the cohort admission rate. This is obtained by monitoring a cohort of children born in the same year and counting how many are admitted to school successively at 5 years of age, then 6 years, then 7 years, etc. By following such a cohort, we obtain a more accurate view of real admissions and can ensure that we have arrived at a figure for total admissions. Chau, T. 2003. Fundamenals of Educational Planning - 72: Demographic aspects of educational planning. Paris: IIEP.