An age group was a classification of individuals based on their age.
Both Miles O'Brien and Jake Sisko placed in the lower third of their age groups for mechanical aptitude. (DS9: "Paradise")
In 2370, Garak ordered the computer of a runabout he and Doctor Julian Bashir were travelling in to create a data bank that cross-referenced the age groups, sex and year in which Cardassian orphans were adopted. Bashir countered the order so that he could ask Garak what he was trying to figure out. (DS9: "Cardassians")