This comparative review of age as a protected ground in discrimination law explores the underpinning questions and themes related to two main dimensions of age discrimination. The first dimension is structural, economic and labour market driven, whereby age is used to allocate a range of rights, obligations and benefits within society. The second is the social justice and equality dimension, in which age is understood as an aspect of individual identity that is worthy of protection against indignity or detriment. The review then considers the law on age discrimination in a number of jurisdictions, the EU law, the UK, Sweden, USA, Canada and South Africa, and assesses the extent to which the underpinning questions explain the developing case law.