Hi Martijn,
Might be a good idea to use a non persistant entity for the user input. You can create this entity with a birthdate input attribute before opening the pop-up and link it to your person object. The non persistant entity will then be the parameter for your pop-up.
The user can then fill in the birthdate. In your save microflow you can retrieve the person object over association or from Database(depends on your case) and then check the birthdate.
For more info about persistability see: https://docs.mendix.com/refguide/persistability