First, go to your Invitation Options
page and turn ON the setting for "Track adults, children, and infants" (in the Advanced Features
When your guests respond, they will indicate if they are bringing others with them and can mark whether those guests are adults, children, or infants.
The age information will appear in a new column on your Manage Invitation
page. The totals are kept at the top of the page just under the 'RSVP Status' section.