Yep, we’ve seen similar issues and it would be great if we had more control in HubSpot of organizing ages based on birthday. Our only solution is that we typically create a custom property with a date picker and then use another custom calculation property to add days from the date to generate an age number. Then we can filter pretty decently, but it’s a fair amount of leg work for something that should be more simple.
Similarly, we actually created a module that acts as a verification gate for any HubSpot Website or Landing Page. We use a custom cookie that you can control to block the page until the opt-in and verify they’re over 21. Have a look if this sounds like something that might be useful to you: https://marketplace.hubspot.com/products/begin-bound/age-verification-gate
Happy HubSpotting!