Weird DATEDIFF logic

I don't know what I'm doing wrong, because I'm sure I've used DATEDIFF in the past and it works fine.

Now I want someone to enter their DOB (date of birth) in order to calculate their age (total number of years and total number of months), so a  5yo is 5 years and also 60 months.

I've tried on 2 different computers, 3 different versions of Tableau, starting from scratch every time and there's something I'm missing but I don't understand what.

If I set the DOB to 1 month back from TODAY() I should get the answer as 1 month old, but it's just not doing that at all.

I'm guessing it's a simple mistake on my part, I just can't figure out what it is.