No problemo! Just leave it blank, because the age entry is optional for our birthday cards. The Vintage birthday cards will display a balloon on the spot where the age is normally displayed. The Modern birthday cards will simply not display the balloon with the age on it.
The Anniversary cards also have an optional age. On the Vintage Anniversary cards, a rose appears on the spot where the age is normally displayed, and on the Modern Anniversary cards, the age on the heart is removed.