Age is a protected characteristic under the Equality Act 2010, and is defined by reference to a person's age group.
An age group can mean people of the same age or people of a range of ages. Age groups can be wide (e.g. people under 50), or relatively narrow (e.g. people in their mid 40s) or relative (e.g. older than me).
The notion of age group is rooted in chronological age, but some age related terms can have different meanings depending upon the context (e.g. whether someone is seen as youthful can depend upon their role - compare a youthful bar tender with a youthful Chief Executive). Age groups can also be linked to physical appearance (e.g. grey haired workers).