We don't have synonyms for age grade yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are aggrade and at grade.
(n) :
(anthropology) An organized, culturally defined social category of people with membership based on age; in some societies it is entered automatically when one reaches a certain age, while in others it is entered through an initiation rite and/or the payment of a fee.
(n) :
(anthropology) An organized, culturally defined social category of people with membership based on age; in some societies it is entered automatically when one reaches a certain age, while in others it is entered through an initiation rite and/or the payment of a fee.