This custom function has a number of output formats for you to be able to get what you need:
Your calc would look something like:
Age( patients::birthdate ; get(currentdate) ; 3 )
and would return the patient's birthdate in years, months, days
I was going to write something out, but the custom function Mike linked is a much better way to do it.
No need to "reinvent the wheel" so to speak. I always have a few custom function pages bookmarked and look there first. It's been years since I had to write a custom function thanks to other's contributions.