Should be a built-in feature, but anyway I found a solution for displaying client age and it works fine
Step 1: implement a ‘Today’s date’ date picker field which is updated every day similar to =today() in excel. HS support pointed me to this way of implementing: https://community.hubspot.com/t5/CRM/How-to-create-a-Today-s-date-date-property/td-p/389836
Step 2: create a ‘between dates’ calculation field and input the Birthday Date field and the new ‘Today’s Date’ date field.
Voila! the system automatically calculates the client age and you just add it to your view.
This ‘today’s date’ field workaround is also incredibly helpful to any other custom fields where you need to calculate how long it’s been between today and any other event date. For example I’ve used it to know how long our clients have been clients.
Unfortunately I have not figured out how to build birthday reminders because I can’t get rid of the ‘year’ in the birthday date field 