What does it mean when someone says, "she is 20 years my junior/senior." Is there a difference in how these two words are used? Any insight is appreciated!
If you are 40 and she is 20 years your junior, she is 20 years old. If you are 40 and she is 20 years your senior, she is 60 years old. "Junior" means younger and "senior" means older.
In the Midwest AE it is the polite way of expressing age as a disparity if youi prefer not to give an age figure. Instead of saying she is 64 if you say my wife is 5 years my junior whoever asked your wife's age is subtley told "it is none of your business."
I wondered whether somebody "of a certain age" was going to jump in here, Harry. As it happens, this is a phrase I hear fairly often (admittedly usually in the "over 50 set").
Accordingly, to answer your question, Kenny, I would suggest that the phrase is used more often in conversation than in writing although I'd hate to impugn the integrity of your dictionary. I just can't remember the last time I saw it in writing (until this thread) but I do hear it spoken in conversation.