Melbourne Airport has free Wi-Fi, which is awesome. It asks you to provide your date of birth to access it, which is not.
It's not uncommon for free Wi-Fi services to ask for an email address, and most of us know that you can always use a fake address (or add a suffix to your existing Gmail address to filter out unwanted extras). The airport sign-up page is quite explicit about the process signing you up for a mailing list.
Asking for a date of birth is less common, and more dubious. At best, you'll be condemning yourself to lots of "birthday offers" in your email. At worst, you've shared information that might be useful for identity thieves.
If you encounter this kind of request, use a fake date as well. 1/1/1900 should do nicely. It's worth reminding fellow travelling workers about this too (as well as emphasising the importance of avoiding ubiquitous and dubious 'free' Wi-Fi networks and other common mistakes).