It's not just an age thing, I know men of all ages who do it. It's a sexist thing. But it is one of those subtle things: in some contexts "female" is the more appropriate word obviously, in other contexts it is used by men to treat women as less than human. You don't really see people referring to men as "males" in random conversation, yet you hear men refer to women as "females" in random conversation all the time.
And before anyone gets their boxers in a twist about feminists overreacting, it's borne out by evidence. A man referring to a woman as "a female" in inappropriate context is one of the most surefire signs that he's sexist.