If you click that, there should be the different life stages but also an option to edit the days in their current lifespan. I think you have to choose 'MC commands' and then there's an option to change their age. Otherwise, if you really want their birthday to show in the calendar: open MCCC after clicking on your sim, it should show their age and the days passed in their current lifespan near the top (e.g., 'child, 13/25' or whatever). Hm, it might be easiest if you turn aging off for your household and just age them up manually on your chosen day?