on iOS:

1. Within the Gather app, tap your Photo icon on the far right of the navigation bar
2. Tap the Pencil icon to open the profile editor
3. Tap Age
4. Select your date of birth (Note: This can only be changed once)

on Android:

1. Within the Hinge app, tap your Photo icon on the far right of the navigation bar
2. Tap the Pencil icon to open the profile editor
3. Tap Age
4. Select your date of birth
5. Tap the back symbol when you're happy with the changes (Note: This can only be changed once)