As per my suggestion u set two variables that one for 'Year', another one for 'Cyear' and one field 'Age'.
Here the 'Year' means input Birth date for the Baby, which as MM/DD/YYYY, Cyear used for Calculation. 'Age' is the output. Now set like below on a Script,
SetVariable[$$year; Value:Year ((TableName::DateOfBirthField))]
SetVariable[$$Cyear; Value:Year (Get ( CurrentDate ) )]
Now use the Set Script Trigger option > Choose 'OnObjectSave' > Click the above Script Name and save, then Run it.
Hope it works.. Good luck!!