Google Plus seems to be seeking birthdates of the members.
Google highlight two advantages of providing your birthday details to the platform:
1. Friends and family can wish you a happy birthday!
2. Visit the Google homepage on your birthday for a surprise!
This panel seems to be appearing for folks whose Google accounts do not have complete details related to this specific field. Google added about this feature of Plus:
When you sign up for Google+, we’ll ask you to provide your birthday so that we can provide you with features like age-appropriate settings when you use Google services and personalized content. If you already have a birthday associated with your Google account, you won’t be prompted to enter your birthday. By default, your birthday (day and month only) will be visible to people in your circles. You can always edit who can see it on your profile.