Age Strata: consists of a classification of people into different age groups. Age strata or stratification is commonly used in census, policy formulation (for old-age care, juvenile care, etc.) It is useful in determining how a certain age strata can help the society or needs help from the society and what policies are required to achieve that purpose, for example, old age homes for people in strata (60+ years) and schools for strata (5-18 years), etc.
Age Group: a collection of people whose age falls within a particular band, say between 30-35 years, etc.
Cohort: is a group of individuals who share some common aspect. Examples of cohorts may include people born in a particular time period (1980-85), working at the same place (Walmart), working in similar profession (secretary, etc.), experiencing same event (present in car crash, birthday party, etc.)