Got to the bottom of this one with @EuanGamble so thought it worth sharing.
My form has a Date control to capture Date of Birth and I wanted to auto calculate the age rather than have the end user fill it in. How did we (and I say we, it was a team effort) do it:
Start by creating a variable:
Use the Function dateDiff then add the form control for Date of Birth followed by Context value Current Date. Variable looks like dateDiff("TotalYears",[Form].[DateofBirth],[Context].[Current date])
I then need to create a rule to ensure my Age control is populated when I input the DOB.
If Date of Birth is filled, then Age value is <my variable>.
Finally, set my Age control to read only on the form...
And when I put my date of birth in it shows: