Why do they say never ask a man how much he earns, and never ask a woman her age?

We restrict our selves asking people things they cannot change and can be judged for that.

You cannot change age of a girl, which is a sensitive issue. Some (haughty) girls are ‘sixteen since sixteen’. Why break their illusion of being an old woman!
Men also cannot change their income in a day.A man’s worth is often perceived by the number of zeroes in his bank account.They often lie about it also. Why take a rich man’s tag from someone!

These things are associated with our pride (read false ego). We must handle them with care!