An age group was a group that one fell into based on one's 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, gender 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")