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In my experience, Alteryx effectively treats Booleans as numbers, True is -1, and False is 0; numbers as Booleans, 0 is False, and any other number is True, based on the operator used, or the specified data type.
So the statement of "21<=[Age]<=45" will be effectively evaluated as: "ToNumber(21<=[Age])<=45", and I would expect always evaluates to True in the end, because both 0<=45 and -1<45 evaluate to True.
The operators in Alteryx http://help.alteryx.com/current/Reference/Functions.htm#Operators_ are all usable on both numbers and Booleans, and some operators can be used on other data types.