If I'm understanding your question correctly one way to do this is to create an embedded data variable in the survey flow. Normally I like to initialize my variables so prior to this example I would set and embedded data variable to create the variable "Age Group" = "None" (this step is optional)
But in your case you would use a branch to select your age conditions (say Ages 18 - 29) and then under that branch set "age group" = Age 18-29
Similar to the example below

thank you. I'll try it out