(Poly)Nominal to Numeric: Keep Value

I do not want to recode nominal datatypes for coding in machine learning, but simply take the nominal value and convert it to it's numeric equivalent.  I often do this in my teaching to show how if a data type is read incorrectly, we can convert it to a new type.  From what I see, the options are more inline with converting for modeling purposes, which is great, but if Age, for example, comes in as polynominal, I dont want to recode the age of 21 to 0 given the range of values. 

Simply, I want the Age to be converted to numeric 21, an age of 34 to be converted to numeric 34, etc.