I can't think of a millennial that would be caught dead with a Hotmail account.
However, I do agree it's age. Probably advanced age rather than young age.
The real problem is that we are allowing "AI analysis" to allow people to discriminate upon something (Hotmail ownership) which we would NOT allow people to discriminate on (age, in this instance) outright.
I guess one worry is that you have multicollinearity, you might get a “double counting” effect on the same signal. But, you should be able to see that in a proper model.
The danger with allowing these kinds of "AI" models is that they will become instances of parallel construction: "We don't want to insure people who are <X>, so construct a model until it coughs up a surrogate to <X> at a 95% probability."